the thing could be running out of fuel .. check for good fuel flow through the entire fuel pathway...
Inadequate fuel delivery will make the engine slow or die until more fuel has a chance to slowly trickle into the carburetor.
The rate at which a partly clogged fuel system slowly refills the carburetor bowl will determine top speed.
And the bike may run excellent below that speed.
From your description (no exhaust) it seems the engine is actually dying sometimes. But it otherwise "runs super good" ..
A whole lot of problems can limit top speed (clogged exhaust, closed choke, throttle cable adjustment, various ignition troubles, etc. etc.), but the things that would actually kill the engine at wide open throttle are very few .. i can only think of one at the moment.. a fuel restriction somewhere.
That bike is trying to tell you something in a nice, gentle way.. it needs a thorough tuneup. Listen to it.