if its a multi speed chain drive, then no.. i wouldnt suspect the trans.. maybe the clutch. If it is a drive problem something's slipping. Thats a big 'IF'.
You would know it if you lost a gear or a bearing.. from the noise mostly. But clutches usually dont just go away without warning. They work their way up to it over time.
in my opinion it's carburation. something like a piece of junk got sucked into the carb's main jet, partially blocking it This would cause erratic carburation and erratic top speed.
Electrical problems a bit more rare so thats farther down the list.. although examining and testing this bike with a brand new spark plug will not hurt.
you gotta get down and examine everything. Whatever it is can be fixed, but not until you find it.