I want my potatoes to be baked within an hour, but I forgot to preheat the oven. Should I put them in while preheating or not?
I'm going to assume 'baked potatoes'[1] to be 'jacket potatoes' in the absence of any other evidence.
As already mentioned, putting them in a cold oven as it heats might win you five minutes, but not much more. It's very slightly more efficient overall than waiting for the oven to be hot then opening the door.
Two additional methods of speeding up jacket potatoes.

Put a metal skewer through the longest centre, then bake in the oven.

Alternatively: Split the task, half microwave, half oven. Prick the skins with a fork first, or they may explode in the microwave. 15 mins* at full power, then 15 mins in the oven is a 'fair guess' at how long it will take. This, of course will vary depending on size/weight of potatoes, power of microwave &amp; temperature of oven.
*I think the last time I ever actually did this, I had an 850W microwave. Reduce time accordingly if you have a more powerful one.
Though the mic should manage to have heated the centres in around 15 mins, you can still use the skewer trick once they're ready to go in the oven. The skewers would still be useful to test the centres are cooked in the mic before you transfer.


The essence of the microwave method is to get the centres cooked before you transfer to the oven, to add back some flavour.
I'd weigh up which method to try based on how much time you have. Skewers will bring the time down by maybe a third, so that might be borderline for your hour from cold. Mic + oven &amp; you're down to half an hour or so, but the result isn't quite as good.
I suppose you could still use the microwave as a starter, even if your result is to be 'roast potatoes' ie, peeled, chopped, done in hot fat. It will be a lot messier, but should still be feasible. I've never tested this, so timings &amp; quality of result you'd have to find out for yourself;)
[1]Addendum
This may be cultural/linguistic, but to clarify from a UK perspective, these are baked (or jacket) potatoes…
Comments suggest the same descriptions are true for the US.

…and these are roast potatoes