Do you by chance have the three point in the full up position? That can put it into the bypass mode if your rock shaft is incorrectly adjusted. Try lowering the three point just a smidge and see if that helps - assuming your hydraulic reservoir is full.
I only use mine for snow removal on our driveway at home, but I just avoid dumping unless I have a pile that needs to be moved somewhere else. Every pass I just raise the bucket when I get to the end of the driveway and push the pile up on to the grass. No need to dump every pass, that would take forever.
I move snow the same way and use Edge Tamers (3) to avoid ever moving gravel or grass. But snow isn't heavy like dirt or gravel when we're 'loading/dumping' it.
If I get anxious and shove the stick all the way to regen/fast dump it often goes slower than when powered. 1800 rpm does it but often below 25F I'm done before hydro is fully warm. (HST helps)
Anyway, we just deal with slow sometimes, and I've learned this is about the worst of them.
"Slow when cold" tractor hydraulics are often a clogged sump strainer or hydraulic filter combined with trans/hydraulic fluid that is too viscous at cold temperatures.