I had a similar problem last year. It was my first year with a mesh safety cover.
I had to backwash every 10 minutes. What worked for me was taking the filter apart and cleaning the grids. Once I put it back together the filtering was more reasonable.

To avoid the problem this year, I bought a black opaque cover - I don't know what this is called - it floats on the water like a solar cover but keeps out all the sunlight. THen mesh safety cover goes on top.
I also add 2 gallons of bleach into the pool (just dump it in) during Feb, March, April, and May. But I wouldn't recommend that if you've got a liner.

Good luck.