Pump sizing for filter

I have a 10,000 gal gunite pool with a Haywood 100 sq ft cartridge filter. The pump is a Speck with a 1.5 hp motor, 2 speed switch. The pool does have a spa attached, but we do not use it.

I just installed a new cartridge and my pressures are as follows:
High Speed - 10.5 psi
Low Speed - 1 psi (actually almost looks like zero)

I don't seem to be getting the filtration that I expected. I was told that low speed was better as the filter can trap smaller particles due to decreased force.

I am considering one of two things, upgrading filter, or resizing pump to a 3/4 or 1 hp. Which would be the best way to go?