I have an older Delta single handle shower scald guard. Water pressure from the cold side is fine. When I turn it to hot it slows to a trickle. After a minute or so it picks up slightly but it is then hard...
It sounds like it has mineral deposits in the hot water side. Try taking the internal valve parts completely out of the fixture and examine them. If there is any scaly (light grey) deposits, that would be the problem.
The solution is to soak all the parts in vinegar, which will dissolve the mineral deposits. It takes awhile to dissolve what has taken long to build up, so heat the vinegar, or put it all on a warm, not hot, surface to keep it more active.
I had this problem, and after cleaning it a couple of times with vinegar, I changed out the Delta to another brand. You could also use a water softening unit, which replace calcium and other metals with sodium, thus eliminating the mineral deposit problem.