20 PSI is about 46' of head but only on the return side. Suction side is probably at least 5' of head. That is 51' of head and according to the head curve, you have about 25 GPM so not much flow rate at all. But the head loss in your plumbing is extremely high for the given flow rate (~5x the average pool with 1.5" plumbing). So I suspect there is some issue with the filter or other parts of the plumbing.

Is 20 PSI the clean pressure and has it always been that high?

Have you backwashed/cleaned the filter recently?