It sounds like something to do with fuel or air..
If the bike's only problem is lesser top speed but otherwise runs normally, something about the amount of fuel available or the air:fuel mixture at wide open throttle is out of whack.
A little piece of junk in the float bowl could be partially blocking the main jet orifice. Or the stream of fuel refilling the bowl could be restricted.. In those cases top speed will be less than otherwise.
Air restriction could be from a partly closed choke, or maybe a problem that reduces maximum air intake through the filter and into the carb.
If the bike hasn't been ridden in a while, go through the fuel system and make sure everythings clean and open.
Check for loose nuts and bolts that might cause air/fuel/compression/vacuum leaks. A general tuneup and examination (with particular attention to things related to carburation) might reveal the trouble.
_the throttle seems to shut down at 25_
My guess would be that if you did a plug-chop at top speed the plug will show a rich condition, indicating air restriction.
If it were leaning out you'd be more likely to sense lots of power coming on around it's top speed.