Maybe look underneath and find out what type of blades it sports and then...
If it has mulching blades try some high lifts.
If it has high lifts try some mulching blades.
On another note and for comparison purposes it takes me about 45 minutes for one acre with a 60" Ztr, I usually keep the speed down to around 4-8 mph and that's double cutting (2x pass) everything.
I only use high lift blades.