Mutual intelligibility is a spectrum, not a binary. How well people will understand each other will depend on how hard they’re trying to be understood, and some language pairs will require more or less effort than others.
I’ve trained Brazilian lab techs with the equivalent of middle school educations, neither of us speaking the other one’s language, but each of us talking slowly, overenunciating, and doing a lot of point-and-smile; thankfully, a lot of the gestures we use are the same, even more so than with Italian. I wouldn’t dare do that if it had involved actual chemicals, but it involved a computer using a training system - we weren’t going to make anything explode. I was using the most Academic version of Spanish I can.
Where do you come up with that head start? I’m not even sure what kind of linguistic separation do you expect us to have; I think you’re missing a pair of important details: first, many Latin American accents have a counterpart in the metropolis. People from some parts of Colombia sound like they grew up 10km from where I did; people from one of the Canary Islands who move to the Peninsula need to get used to being mistaken for Argentinian. Second: the influx from the Americas back to the metropolis was a very thin stream until very recently (intentional collaboration such as RAE and OTI, movies, books, music), but Spain has always sent people and media across the sea. People my age learned to read with the Cartilla Anaya (published in Spain) and with Mafalda’s hyper-popular comic strips (Argentinian) on both sides of the Atlantic.
This. I had to take a phone call from a guy from New York for my job, and hand to Og I only picked up about two thirds of what he said.
I have a pet theory that in many cases a native speaker may be harder to understand, at least for someone not from that area; nobody has trouble following the Indian fellow with careful Received Pronunciation, whose native language is not English, but the same cannot be said for a tipsy Glaswegian/Kiwi/(U.S.) Georgian.
I found that the most striking difference is in the cadence of the two languages. Portuguese is spoken in a musical, “lilting” fashion (reminiscent of Italian), while the tone of most Spanish dialects tends to be more even. Also, there are relatively few vowel sounds in Spanish, and they are “pure,” while Portuguese has many variant vowel sounds, and vowels sounds may be more slurred.
Compare these guides to Spanish and Portuguese vowels. Note that the Portuguese page doesn’t even get into diphthongs.
Here’s a video of Spanish speakers struggling to pronounce Portuguese words. 
It’s not that all Romance language speakers can understand each other. It’s that there are language continuums, where someone in village A can speak with neighbours in village B, even though they have some differences, and villager B can speak with their neighbours in Village C with a bit of hand-waving, and so on. But if you put villager A next to villager Z, they are so far apart on the continuum that they can’t communicate, even though at each intermediate step, communication was possible between each neighbouring village.
Portuguese and Spanish are both Iberian Romance languages, with more shared history than Portuguese and Romanian, which are probably at the opposite ends of the Romance continuum. Portuguese still apparently shows some Celtic influences, while Romanian has Slavic influences, and they were isolated from each other for a long time, unlike Portuguese and Spanish.
For instance, I don’t know a word of Spanish, but I can do simple communications using French.
A couple of years ago I was in a laundromat in Florida and there was a Spanish-speaking couple who couldn’t understand the instructions for the electronic dispensers and payment cards. A guy from New York who only spoke English was trying to help them, using simple English, but it wasn’t working.
I asked in French if they spoke French, and they said “No” in Spanish.
But I just kept speaking in simple French to explain the vending and booking machines, and they got it and were able to do their laundry. French and Spanish were close enough, even though we technically didn’t speak each other’s language, while English and Spanish just didn’t work.
I also once had a conversation about language continuums with a German speaking Swiss at university. He said that until he came to the US he hadn’t realised just how isolated English was in the Germanic languages continuum, because of the huge French overlay and the drastic changes in grammar from continental Germanic languages. He said he could converse with Dutch, Flemish, Plattdeutsch and regular German speakers, but he thought English-speakers couldn’t? I told him that was my experience, trying to speak with other Germanic speakers.
Spanish and Portuguese colonies were started in the 16th century; English colonies in the 17th. It may not have been exactly 100 years, but for this purpose it’s close enough.
Simple physical separation from the Old World. The fact that the interactions between people in the New World and Old were limited.
One certainty about languages is that living languages change. And two separated language communities, starting with the same language, will drift apart because of that change. One will borrow words that the other doesn’t, pronunciation of vowels of one will shift and the other not, inflections may be dropped by one and not the other, etc. Given enough time, even with some interactions back and forth, they’ll eventually be speaking non-mutually intelligible languages.
However, it takes hundreds of years to get to the point where they’re different languages. The New World languages haven’t been separated quite long enough for that to have happened. And because of greater communication and transportation today, they probably never will.
I would also suspect the mutual intelligibility to be a bit asymmetrical. There are about 250M Portuguese speakers in the world, but about 500M Spanish speakers. In general, one would expect Portuguese speakers to be more exposed to Spanish than vice versa.
Yeah, I was being a bit facetious. But having formally studied both Spanish and Portuguese, I can tell you that it often seems that they were children separated at birth. You would have better luck speaking French to an older Portuguese, or English to the younger folk, as those were the second languages taught in the schools in the respective eras.
Colibri mentioned that Portuguese has a lilting sound to it. The Brazilian version of the language certainly does, but I’d have to question that descriptive for the Iberian version. The Brazilians are, in general, an effusive and expressive people. Present-day Portuguese (the men, in particular) tend to talk inside of their mouths, for lack of a better description.
As far as English spoken in England, the US and Canada vs. Portuguese spoken in Portugal and Brazil goes. The difference I see is that the English came to America and “took over”. The native population was virtually killed off by disease and slaughter. The few indigenous tribes that survived had no effect concerning the language that was used in the area that is now known as the US and Canada. The sheer numbers of people who arrived from Europe after “contact” completely overwhelmed the people who lived in America and their language.
Did this kind of process happen in Brazil when the Portuguese arrived? Did so many individuals arrive from Portugal that the local population and their language was wiped out? I don’t think so. The “Portugueseization” of Brazil I believe was quite slow and was concentrated in the large cities where local, indigenous control and the local language was pushed aside. But in the countryside, the native population was largely unaffected by the introduction of the Portuguese language. I believe local native languages survived the Portuguese invasion and are still used today in some areas. How far off base am I with these musings?
It’s true that, compared to the US, indigenous cultures were more integrated into the colonial cultures in CERTAIN parts of Latin America – mainly, where those cultures already had urbanized, sophisticated civilizations (central and southern Mexico; Guatemala; Peru, Ecuador, and Bolivia). Brazil, not so much. (African cultures had a bigger impact on Brazil and its Portuguese dialects, especially along the north-central Atlantic coast).
In Brazil, the Portuguese concentrated on developing coastal ports. Indigenous cultures were indeed “pushed aside” or killed off, similar to the US, except in the vast Amazon region, where many were left alone until recent decades.
In any case, indigenous languages haven’t had a great impact on European colonial languages, ANYWHERE in the Americas. Yes, many Nahuatl (Aztec), Mayan, Quechua, etc. words have become part of standard regional varieties of Spanish, but no huge influences on grammar or even vocabulary. Many, many people are bilingual, in an indigenous language and a European one. Quechua has about ten million speakers (in the Andes Mountains), yet Peruvian Spanish hasn’t been affected much.
I think it’s because the language change of which you speak happens only in certain circumstances – some pre-Indo-European substrate’s influence on Greek, say (even that wasn’t that huge), or a Sprachbund of otherwise unrelated (or less-related) languages adopting some similar traits (e.g., Slavic influence on Romanian – again, not THAT huge), or an invasion where the men marry a lot of local women, but don’t impose their language politically over a big territory, and the kids pick up on the simplifications and mistakes of their parents’ conversations, i.e., partial creolization (as the Danes did in England in the 800s, which is a main reason English is so different from other Germanic languages – but it’s not THAT different).
They were continuous and widespread, though, and the areas of largest contact have, for 500+ years, been those with the highest population densities; the majority of people have lived in the areas that had more contact. It’s not as if there was an initial movement 500 years ago, and the people who moved did so shaking the dust off their sandals. The movement has been continuous, the influence has been continuous. There has been an asymmetry to it, but that’s a different matter.
You seem to think of a situation akin to what happened to Latin after the fall of the Roman Empire - no, the situation is akin to that of Latin during the Roman Empire, even after the American colonies went their separate ways.
As a Portuguese as second language speaker who was based in northern Brazil who met some Portuguese people, I could understand both just fine.
The most unintelligible person I ever knew when I first met him was from Rio. The accent is VASTLY different between the north and south.
There is no way a Spanish person would understand a Brazilian (trust me, I tried). Maybe a word here and there but the words are different and the pronunciation of most of even the same words are different.
I could understand a spanish speaker a fair bit, but only because of high school Spanish and 6 months in Mexico.
We had a Brazilian exchange student and he could communicate with the Columbia exchange student across the street. That is before they both learned English.
It’s your word against, mine, querido. I suspect my Spanish is a little bit better than your Portuguese, and it also depends on what the subject is as well as, like someone already mentioned, how much both parties are interested in communicating. The supermarket cashiers I had problems with, but those women were so surly I don’t know how did the till itself understand them; the people at the company cafeteria no problem; the lab techs, it wasn’t easy but we got by. None of them spoke Spanish or English (their bosses did generally speak Spanish as a second language, English as a third or fourth).
“Getting by?” The very first time you met them? I’m not talking about struggling and getting stuff from context, I’m talking about true understanding. There’s no way you’re going to understand that spanish “dog” (perro), is the same as Portuguese cachorro (roughly pronounced ka-show-hoo to those of us not in the know - not even intuitively pronouncable) without barking and pantomiming like and idiot like you’d do talking to a german. Depends on the subject? How about everyday stuff like animals? That is not an isolated case, not nearly.
Talk about a bad example.
Cachorro means puppy dog in Spanish.
And you know the pronunciation is completely different.