From personal observation, I can say that:
- off road/enduro tires tend to decrease the topspeed,
- worn off road/enduro tires tend to reduce it even further,
- dirty/non-lubed chain makes things even worse,
- incorrect valve clearances is a disaster,
- add some misaligned brake pads in contact with the caliper, or
- the rider's unconscious habit of squeezing the front brake at all times and by now you have already lost more than 10% of your topspeed.
- Raised or modified windscreens may have a negative effect on topspeed,
- Too soft suspension (particularly the rear) is not too good, either,
- Weight is always a drawback, trust me I know (being heavier than a common offroad bike usually sucks).
- Panniers often ruin the aerodynamics,
- Passengers are good for chat, bad for speed
- I suppose overall reduction by now is some 15%, which is close to what you get from your bike.
If you get more scientific, I agree that engines work better in cold due to increased compression. Whether it is sufficient to eliminate the negative effect of increased draft of your bike+rider due to more dense air, however, is beyond my knowledge.
Whatever. Before messing up with your carbs or worrying yourself to death, I would suggest you to check all the issues above.
PS: Right after having my 2004 KLE fully serviced, with no negative elements above, I was overtaken by a 2005 KLE, which is restricted and had a passenger. So I may actually be talking crap!