Why do you engage the blades at “low speed”?
It sounds as if the belt is glazed. Check all of your pulleys, idlers, and spindles. The belt may be on the wrong side of a keeper.
A side comment on engaging the blades at low engine speed:
I had a 42" 16 (or 18?) hp 2009 Western Auto Wizard mower (I forget which of the 2 mower manufacturers of those days it was made by) - bought it used and used it for many, many years on a couple of rough acres.
Anyway - I was having to replace the very long blade drive belt 2 or 3 times a season. I would always engage the blade with engine running at relatively low speed and then speed it up to full speed to start mowing simply because I figure that's less stress on everything - it's just my habit of thinking that way in general.
I noticed that my wife always put the engine at full speed before engaging the blades. I got to thinking that that high-load shock was possibly killing the belt - the way the belts were failing (short sections were separating) supported the theory.
So I mansplained my theory to my wife (can be dangerous, but I took the risk
) and asked her to drop the engine speed before engaging the blades. She'd forget every once in a while, and I'd remind her (
). Telling y'all all this to say that the belt started lasting well over 3 years between changes once we got the slow speed blade engagement thing established. Saved money and maintenance time.