I'd say that the missing belt(s) is your problem if both are missing, but if it's only one, and the smoke was coming from below the air-filter area; your turbo could be at fault. A failed turbo bearing could produce all your above problems (more noticibly when/after driving hard).

I have a BMW that exhibited similar problems when its turbo bearings went, though I noticed smoke in the exhaust gasses too (though this would be nothing new for a diesel and probably wouldn't be noticed anyway).

Sincerely

ATLD