If it was a sudden onset, it may have been dragging something, but I'd expect you'd have heard something. Otherwise, like the other suggest, tire pressure, fork/shock spring preload and/or oil level in the fork, loose-ish swingarm bearings, etc.
The airheads can still corner at pretty good speeds - the whole thing is that one has to be smoooooth about it. Chopping the gas or cranking the throttle midway will be rewarded with significant suspension settling or rear end jacking, which will upset your line through the curve. The monolever and paralever bikes are much less prone to this.