Most always indicative of the filter capturing debris (algae) and slowing the flow. Does your psi go up when the flow slows?
Pump running, do you have air in the pump basket after you backwash?
Right now, things seem to be running good - PSI is at 4, RPMS are at 1900. Yes, I think when things slow down my PSI jumps to around 8.
Priming PSI is about 18 which I think is the true PSI you are supposed to look at on a variable speed. That never seems to change. Always 18.
Last summer, unless priming my psi always showed 0.
New pressure gauge.
A few weeks ago, I had air in the basket no matter what, you could visibly see bubbles coming in the basket and the water level in the pump would drop to 1/2 within 30 minutes, as well as bubbles in the jets, even at really high RPMs (2500+), there would be bubbles coming in the basket but at that speed the water level wouldn't drop so dramatically. There would be air while back washing and immediately after. As mentioned... I never fixed this... it went away after an enormous amount of rain and the new problem of everything slowing down started.
I haven't saw bubbles in the basket for a few days, and only see bubbles from the jets if I drop the rpms super low, like 900. Which, I actually read can be normal for variable speed pumps, too

There is absolutely no bubble in my basket today at all, it looks crystal clear.