#1 How fast does it shred, how many feet per minute (fpm)?
I shredded 12-15 pages at a time - had zero problems - it shredded as much as I shredded - 10 years of paperwork- 4-5 hours at a time continuous.
#2 Can this deal with window envelopes or does the window melt and gum up the blades?
No, window envelopes do not melt or gum up the blades! As long as you follow the instructions on when to apply oil to the blades, it always keeps on doing its job for me! It is a wonderful machine!