No shit, those CDI's do wear out. the pickup stops picking up and they start limiting or just getting slower and slower. I dunno why/how, but they're really shitty CDI's. I'd guess it's just missing more and more often, but it's high enough rpm that you're not hearing it miss really, you just lose a growing percentage of your power.
I've had a few - some were busted within a few hundred miles, all needed some parts replaced, whatever it was - coils, pickups, light coils etc. usually the box was the first to need replacing. Longest one was I had one I kept piecing together from other ones when parts would fail until it finally was a solid working set, lasted 5 years.
but after 5 years it did die, points aint gonna shit out after 5 yrs and this is the best of those CDIs out of probably 10 or more. pickup had died, but with a new pickup it regained like +5mph.
Venkat had one on his magnum, at one point it just kept getting slower, like way slower - new pickup restored it
Not syain it's always the pickup, but that's been in my experience a cause for gradual decline in the performance from those shit CDI's.
I got a new one on my sachs now, runs great but I'm still gonna swap it to points cuz I don't trust it. shitty CDI is shitty.