I have a Sta-Rite SM8150 cartridge filter. Prior to this year, after cleaning the cartridge the PSI would start off at about 20 then eventually rise to about 30 and there was no major loss in flow. The output side of the filter plumbing goes into a spa and then a spillway. So I can observe the flow rate. This year, when I clean the filter it again starts off at 20 but rises much more quickly to 30 PSI and the flow rate is noticeably diminished. When I look in the pump filter basket it is not filled up. At the beginning of the season I attributed the basket chamber not being full to air leaks. I fixed those. When I run at low PSI the chamber never empties seems to have air in it. However as the pressure builds there is more air in the basket chamber. That said, the air minor air leaks I fixed this year were there last year but I don't recall having this problem.
Could it be that the portions of the filter have become impermeable and that is causing the faster rise in PSI and the restricted flow? At roughly $500 to replace the cartridges, I'm hoping to avoid doing so.