You are quite the disciple.
Don't you think that nature of the symptom is quite compatible with some debris that can be washed out rather than a shortcoming of the unit.
Yeah, maybe. I was just goofin' around, but I have to say that Fluidmaster certainly doesn't write its instructions for the novice. In contrast, the latest Korky video is super-detailed about the teensiest things, like to reinstall the R528 cap you should point the nipple a 3 o'clock, push down, and then turn right, and to be careful that you don't unlock the thing. Doubtless as a result of fielding questions from the really-clueless, but I appreciate their effort.
On the other hand, any number of youtube handy-hack "experts" put the stupidest stuff in their videos. I just watched one replace a fill valve and say that it was set to shut off at the right place when it shut off as the water just began to run over the top of the overflow riser. It turned off, water sloshed down the riser, and the valve held. Oy. Another person who clearly never bothered to read the directions.
We're really only guessing here when all we have is "it's running slowly". But if he decides to replace it, I'm plainly a fan of this choice, mostly for its simplicity in installation and repair.