If you're mulching, the deck "rake" (front-to-rear level) needs to be 0-1/8" low in front. Even the, you get better cut quality if you overlap a little. Must run slow travel with mulching, really slow. Blades must be VERY sharp (razor?) AND the tips MUST NOT be rounded, it needs to have a sharp 90 degree point at the tips; if rounded (at all), cut quality suffers drastically.
When leveling, measure the rear of one of the side blades and the front of the center blade.
A lot of that also applies to side-discharging, but for mulching it's imperative.
Kubota sets them at 1/4" or so lower in front and you get 3 stripes, one for each blade. 0-1/8" down in front will give it one wide stripe that looks better, IMO. 1/4" is allowable but I like perfection. The downside is that at zero, it takes more power to cut. Not a lot, but noticeable. Some customers are really picky and those are the ones that require attention to every detail!
Slipping belt will affect cut. So will loose spindle bearings, Out of level (front-to-rear and side-to-side), bent deck shell, bent blade, dull or worn blades, cutting too low (affects under-deck airflow), deck plugged underneath with wet/stuck clippings, loose spindle pulleys, and a laundry list of other things.
I know mulching wasn't mentioned but what I've learned by mowing cemetery yards who wanted no clippings also works for side-discharging, and works VERY well. Your travel speed will vary. Your mower and your grass will tell you how fast you can or can't go. On my yard (country yard full of weeds), I can cut at 10.7 mph...which is the speed of my G1900...however staying on the seat at that speed is challenging particularly with 4 wheel steering.