speed loss

so,
after searching, and asking people, the dad etc. i've ran out of ideas, so its time to go to the guru's
its a 99 curvy
im restricted, thats a big part
used to be able to comfortably cruise at 85/90, (wrongly)and even did 105 on an empty a3 on easter morning! but these were impatient days and newfound powerrrr
i generally dont go that quick now, but recently when i do, i've struggled to exceed 80, where i used to be able to cruise at higher speeds i now have to drop down gears and pin it, yet it still takes a long time to crawl up, and im not gonna thrash it, i just want to know why it suddenly takes so much more effort to do what used to seem so much easier. (the restrictors have not been removed in all this time)
i changed my exhaust, thought that might be a factor, but testing both with no difference ruled that out,
so im wondering if for some reason the restrictors have become more effective somehow?...the weather?....not so frequent high speed road jaunts?.... im out of ideas
anybody have a clue?
mike

It could be lots of things causing it, process of elimination. Have the plugs been changed lately? Air filter nice and clean? Chain adjusted correctly?
Ta.

are your brakes binding?

Fresh spark plugs have been put in, carbs and air filter have been cleaned out, chain and sprockets are due for replacement, but I didn't think that would make such a difference like that?
And I don't think my brakes are binding, saidled through the MOT the other day without a glitch

tyre pressures?

nope, i consistently check them theyre fine