Exmark still squealing with thick grass

[robby]

So I replaced the belt on my Exmark but it didnt help. I'm still losing blade speed and getting a squealing noise when trying to cut grass that is too tall or too wet or at too fast of a speed. I realize that you can not go flying at full speed through tough cutting conditions, but I know the mower used to cut through worse stuff without losing steam. What else might be causing this? The mower is a couple years old and the pulleys, springs, etc seem fine.