If you have a continual air leak INTO your pump, you will also have a continual air lead OUT of the pump. Unless you have some sort of continuous air vent on your filter, this will eventually show up in your pool as DISSOLVED air, that then coats everything with tiny bubbles (usually) or turns the pool milky (rarely) OR you will have a stream of bubbles showing up coming out of a pool return inlet -- it may be a small stream.
So, check that.
But in my experience it is uncommon -- it does happen -- to get an air leak so nicely balanced that it slows, but does not stop, pump flow. More often I've seen small leaks EITHER cause dissolved air but no other problems OR ELSE make the pump nearly impossible to prime, and difficult to keep running.
Another possibility is a clogged impeller. I've often seen impellers in my part of the country, clogged with pine needles (residential pools) or swim googles and toys (LARGE commercial pools) produce symptoms like you describe. On some pumps, people with small hands can feel through the back of the pump basket chamber, into the throat of the impeller, and detect clogs. But often, you have to pull the pump apart to check.
By the way, common causes of clogged impellers are
1. debris that will pass through the basket (pine needles!)
2. a cracked or split basket
3. a missing or mis-installed basket.
One episode of vacuuming with a broken or misplaced basket can be all it takes!
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