I usually wait a lot of time on the very first levels in order to collect lives for harder levels, but I have never noticed this lack of time on hell levels.
My 2 cents are that on the first levels you are pretty relaxed and it seems that time passes slowly (I can usually collect about 4 letters) but on the hell levels you are worried and hastened because of the difficulty.

Maybe someone could test it with a real timer, it could be an interesting theory
