Started noticing the red light on my oven wasn’t cycling on/off. Preheating the oven used to take 15 mins. Last night I checked it with a oven thermometer and after 30 mins it was only up to 300. Thirty mins later it still hadn’t quite reached 400 degrees. It was about 350 and still trying to get hotter.
It’s not like the thermostat is cycling it on/off. the red light is staying on the whole time.
What should I check?
I’d check the heating element / socket / thermostat.
If this is an electric oven with a heating element in the bottom, the first thing to check is whether it has been cracked or damaged in any way. These things do wear out over time so they can fail without it being anyone’s fault, but they can also be damaged if something got dropped or spilled on them. A bad failure will result in no heating at all, but during my days as an apartment manager, I saw this exact scenario a few times. I assume because of an element that was cracked only partway, but I don’t know for sure.
That has exhausted my knowledge of oven mechanics and repair, so if it’s a different model or a different problem, someone else will have to come along. 
I’ll check the element and socket then. Hopefully I can still get a replacement element if needed. It’s a GE stove made around 1988. Been a really good one too. Even the mechanical clock and timer still works. That would all be digital today.
Came with the house and the previous owner left me the receipt from Montgomery Wards.
Yes, oven elements are fairly standard. I just replaced one from an even older stove. Get the model number and search online, and you can find them. I ordered online, it arrived in a few days, and was fairly easy to install.
And the oven heats up partially because the top element is probably still working. That’s used especially in broiling, but also during baking. But it’s usually not enough on its own to heat the whole oven. The top element seems to last longer, maybe because it’s not exposed to spills, etc. as much.
Are there any bright spots on the element when it’s on? About every 10-15 years, the bottom element in Mom’s oven would develop a bright spot and eventually it would become an extremely bright spot and fail with a shower of sparks.
As t-bonham said, they’re usually very easy to change - it may take you longer to find and turn off the oven’s circuit breaker (or drag the range out to pull the plug) and find the appropriate screwdriver than to remove two screws holding the element to the oven’s back wall, and the two terminal screws on the element itself. Somewhere between now and 1970, there was a move to plug-in elements that are even easier to change. Be gentle with the new piece - if you crack the internal ceramic and cause a hot spot, you can lose a few years’ lifespan.
AFAIK, Mom’s oven broiler element is 50 years old and has never been replaced. If your oven turns on the broiler during baking, it’s probably at half-power (120 volts instead of 240) so it’s just loafing along, so to speak.
There’s also a possibility that one leg of your breaker has failed and you’re only getting 110 volts instead of 220. That happened to my electric dryer once, the motor and fan ran, but the heating element didn’t get hot.
This happened to me also with the oven. The inside oven wouldn’t come to full heat and only very slowly, and we could only run 1 or 2 of the top burners at greater than medium heat.
There was one leg of the 110V that was tripping its breaker, the other leg was fine. There was a short in the very old under-gauged house wiring somewhere.
Yes, sounds to me like a half power exactly, just based on gut feeling on how long it takes to get to 400 F…
The “about to fail” degradations should speed up the oven (it fails because its drawing too much power !), and certainly no slow it down that much.