My toilet refills very slow when I flush which I have found many articles about... but! When I turn off the water valve and turn it back on it refills totally fine. I cleaned out the seal under the cap and also the cleaned debris in the tube. Not sure what's going on please help! Was thinking I just need a new seal but it's a bit odd that it works fine when I turn the valve on/off but not when I regularly flush.

Thank you so much in advance for help!

You either have debris caught in the stop valve or you have a loose washer in the stop valve.

Turn off water. Remove the valve stem and tighten the washer. Wrap the valve with a heavy cloth, place a pan under it and turn the water on momentarily to flush the valve. Reinstall the valve stem.

Or, just replace the valve, they are pretty cheap.