So I know this question has been brought up before.
"Why has my meter gain slowed?"
I tested this today and found that it's completely unrelated to how much meter you have or how often you hit your opponent.
All meter gain, for about ~7ish seconds (used a timer so I don't have exact period of time it takes), is dropped to ~.425 (seemed a little inconsistent and varied from around .425 to .435) of it's original value. This happens when you use any EX special or Arc Drive. There are no other factors in meter gain from what I've discovered and I haven't tested if doing a second EX special within the ~7 seconds will reset it, but like reverse beats, I imagine it would.