My 04 JX75 will start just fine, however just recently when I try to engage the blade 2 things occur.

1) it takes around 10-15 seconds for the blade to reach full speed, and
2) once I start trying to cut my grass the blade slows to a stop

The blade lever never disengages and the motor never dies or slows. New plugs, gas, and oil. Any ideas of what could be wrong?

Hi,
Does it make a scuffing sound? Kind of bet it does. The friction disc has fractured and probably been "spit" out. Inspection will tell you whether you can replace the lower half or the whole unit must be replaced. If caught early, the latter will work.
The part numbers are:
GY20711 Lower half
GY90029 Clutch
Peace,
Clarke