The pump that comes with filters is often larger than needed- the Tim Taylor effect!
The dirt that comes back in your pool- is it really fine? Sand filters will sometimes pass really fine stuff, in which case you need to add some DE to the filter to help it out...
Backflush filter
Note the pressure
You can get some DE, add some (maybe 1/2 cup) to the skimmer on your pool and let it suck into the filter
Watch the returns and see if the DE comes back into the pool- if not, your filter is fine (pump may still be oversize, but the filter will work!)
If the DE does not come back into the pool, keep adding DE 1/2 cup til your pressure raises 1 psi
Now the DE will help your sand get that really fine stuff out
Next time you backflush, the DE will come out the waste side... if the fine dirt returns, you'll just have to add more DE!