Yes, this does sound stupid

Easy answer...it's broken.

I've had ovens break on me here at home before. Probably one of the heating elements in your oven, is shot. Call up a repair fellow, no one fancy, just a cheap, repair man, tell them your oven is not heating up quickly anymore and isn't reaching proper temp. and they'll come over and fix it.

Should cost you no more than a $100 to repair it. Peace imp: