Originally Posted by Iride01

I'm not certain I understand how long and hard you are riding per week. If you are riding for more than an hour and a half at a time at a high effort, then both heat and hydration can be a factor. And what you hydrate with and what you do after the ride can play a part too.

If you are riding that mountain bike mainly on paved roads, then you need to get a bike for the intended use. You might be just using all your energy trying to keep that high average speed and not getting good cardio workout because you aren't riding as long. Short rides at max HR aren't as good as longer rides at a high HR.

There is also the issue of sometimes it takes a while to get used to something.