First time I cut our lower section of the property, it was 1s gear Low Range. This is about 7/10th's of a MPH, so it's pretty slow. Now, that's at the rated PTO RPM (2600 or so on my 670). The brush was as tall as the tractor hood (or taller, sometimes). I went slow enough to react to any stumps or debris I couldn't see (too thick to do a walk through effectively).
After that first cut, and the finishing mower work is normally 2nd or 3rd gear low (still pretty slow, but my property is somewhat sloped and rutty).
But, you do need to run at the rated PTO speed for your cutter (or mower) to work efficiently.