They also hate them because after they don’t work, a plumber, i.e. them, is called in to deal with a clog now containing a hazmat spill of caustic chemicals; not just the usual hair, soap, shit, & piss.
IOW, they have their standards. 
No, seriously, it’s dealing w the noxious chemicals in the water behind the clog they hate.
The chemical damage to pipes was a real issue in the cast iron era. Not so much in anything originally plumbed in the last ~45 years since PVC became nearly universal.
In terms of that explanation… how does that work?
I get poor drainage secondary to poor venting, but why the … phase change? … after a certain pressure level sinkside?
Not questioning the answer; just trying to understand it.
I, too, have no idea. Just quoting what I found!
Happened to remember this in the bathroom today.
In my sink, the water seems to start flowing out at once, as fast as the tap is adding it.
The level never rises beyond a sort of half inch swirl.
Just a data point. I imagine the behavior is very dependent on the design and geometry of the fixture (type of trap, length of outlet pipe before it joins to a larger sewer pipe etc)?
Yeah; your way is by far the norm for non-defective plumbing. Which is what makes the thing I’m talking about so interesting.
I think it’s the air in the trap. Can’t rise up through the sink full of water so limits flow through the drain until it gets carried down the drain or bubbles out. Once the air is out of the way the sink should drain quickly.
I don’t think it’s a venting problem. That would result in the sink draining slowly the whole time, or at least much longer before the air bubble is gone. I wouldn’t expect this to happen often in a kitchen sink or anything with a wider drain than a bathroom sink.
So why is there air habitually getting in the trap?
I can imagine a long unused sink having water evaporate enough to get below the trap, but am failing to imagine why ever otherwise. A below air pressure level in the stack at times sucking it out? Why would that happen? Poor venting! 
The air is in top of the water in the trap. That’s the way it’s supposed to be.
Not by my understanding. The bottom of the U should be all water. That’s the point of it, providing the seal from fumes in the stack.
Yes, but the drain will be full of air on top of the water in the U trap. It’s not about air below the trap on the outlet side, that should escape through a vent further down the drain path. The water in the trap and in the sink doesn’t allow the air on the inlet side of the trap to easily escape.
try starting the water very slowly and each time you experience and back up or turbulence back off for a second then start increasing again. I don’t know what that would tell you but I think it would be a clue for someone.
Your major drivers of fluid motion include pressure and “head” or height difference between different parts of the system.
Turbulence vs. laminar shouldn’t make a big difference in the fluid motion, not at the relatively low speeds (or Reynolds number) we’re talking about. For instance, using a pressure washer to blast fluid down the drain would have a high number–but instead the situation is that of (quiescent) water in the sink suddenly draining more quickly.
I think the biggest part of the answer has already been delivered: The height of the water in the sink reaches a critical level, starting the water in the P-trap to really start moving, then the inertia of that faster-moving flow drains the sink more quickly.
That does make sense! I don’t have a real good understanding of laminar flow, but it does interest me.
Inertia will be part of the answer. Possibly the greatest part, don’t know, can’t even observe this happening until I get some Draino.
It is about momentum.
It takes a little while to fill the p trap with water.
Then there is a mass of water slowed down by the constriction of the p trap.
As the head increases the speed of that mass increases.
At some point the speed of the water in the trap is high enough that the flow in the trap is higher than the flow out of the tap. This empties the bowl. Because the water in trap now is still moving the trap has a higher capacity than it had with standing water.
Huh?
There is air on each side of the water in the trap. How does that explain the behavior described?
Again this is behavior that most of us never observe and that that OP has only observed in a few sinks. Unclear if it always happens in those sinks or not but it is an initial period of poor drainage then sudden shift to good drainage.
Inertia makes no sense as an explanation for this only a rare sink behavior.
@LSLGuy once it is draining well if you turn off the water, let it all drain out, and then turn the water back on, how does it drain? Does it do the same exact thing again? Or does it drain well at that point?
Also does the sink have an overflow drain? If so can you test if its path is clear?
As @LSLGuy noted, plumbers also hate chemical drain cleaners because they can be a hazard when they’re working on a clog. I have mixed feelings about them. I’ve had good success using them to keep bathroom sinks free-running. But when I created a massive blockage below the kitchen sink by very unwisely pouring chunky pasta sauce down the drain, drain cleaner failed to clear it because chunks of debris had worked their way far down the pipes. But since the pipes were thoroughly clogged, the drain cleaner ended up sitting in the pipes overnight and most of the next day and causing a leak. It won’t corrode PVC plumbing but I presume that if allowed to sit long enough it can start to dissolve PVC joints. I’m now much more cautious about using the stuff.
There is a ton of air on the outlet side of the trap. It’s a long empty drain pipe and it won’t restrict the flow of water passing through the trap. As the sink fills a bubble of air gets trapped between the water in the sink and the water in the bottom of the trap that will have some effect on the rate of flow. Some of that air escapes under pressure through the sink drain opening before the sink fills and more is entrained in the draining water. That’s how common ‘airlock’ conditions occur in all sorts of piped systems. In this case I doubt it could lock because the water is being pulled down by gravity all the time and the diameter of the drain pipes are rather wide unlike something like a fuel line in a car. So the result is some reduction in the flow rate.
However, momentum (not inertia as I said before) sounds like the major factor. As more water is flowing down the drain the momentum will be pulling it through the whole drain system faster, along with air entrained in the water as it goes. And that’s why an air bubble wouldn’t explain the increased flow rate as the draining continues.
And as I understand it a functioning overflow drain helps flow for that reason. Hence the question about presence or absence of one and if it might be partially clogged.
But this is an uncommon observation. A bubble preventing full flow should be something that occurs in all sinks. Why just these few? What would be different about these ones that make it occur then suddenly resolve?
I remain stuck on dried hair either in the overflow drain or hanging on the stopper, that softens up and reconforms out of the way as it gets wet.
But again, if immediately after draining the same behavior occurs again, that is ruled out.