Certainly the gas pressure used to drop at peak times, it affected our first gas boiler as well as the oven, my impression is that it got better after we went to natural gas, and hence got rid of the gas holders used as reservoirs for town gas?
Not sure about 'lectric
But don't forget that if you really fill an oven, say you normally put 1kg of food in, but at Christmas you put 5kg in. To raise the temperature of the food by 1 degree C then needs (obviously, but we all tend to forget it) 5 times the heat and five times the electricity, so even working flat out an average oven may not be able to maintain the intended temperature as the food soaks up the heat. Plus even in a fan oven circulation isn't as good with the oven full.
If nothing else it's a good argument for pre-heating the oven properly