I can’t say for sure since I don’t use them, but I just think that maybe, that is the source of the problem, so I suggest cleaning it in similar way as I do just to be safe… heck the gasoline might be different too but trying on already bad bearings can’t hurt…
On another story, normally we had much more friction coming from the string rubbing the yoyo body than from the bearing itself, that is the reason why yoyos can spin over 5 minutes flat and still bindable, yet it can’t spin as long when doing tricks. The way I see it, as long as the bearing doesn’t “drag” the string when you tug it up, it’s perfectly fine… unless when you are competing in long sleeper contest, by then of course bearing quality matters.
Btw I get your idea on the yoyo slowing down earlier though, I can imagine the rpm vs time curve may be different, but I have no idea how to test it.