Pressure gauge running high at low RPM, sanity check please

Running my IntelliFlo at 1500 RPM for the 8 hour filter block, filter side gauge is sitting at 12 psi where it used to sit around 7 to 8 at that speed when the cartridge was newer. Cleaned the cartridge two weeks ago so it is not that. Affinity laws say head should drop with the square of speed so I expected way less than this.

Only thing that changed is I added a check valve on the return line after some chemical feeder plumbing last month. Wondering if that is adding more head loss than I am accounting for, or if I have got a partially closed valve somewhere I forgot about. Anyone dealt with a new check valve eating more psi than expected. Before I start pulling unions apart I would rather rule out something dumb first.