I have the same issues, but on a Surface Book 1.
I bought my SB1 when it was first released in the UK. It was the top spec version at that time, 1TB HD etc.
I have used it very very heavily in the last two or so years. I am a freelance consultant / workaholic and I spend way too many hours at work. I have used the SB1 exclusively in that time since it was released.
I think it is a cracking product. Never had any issues until Summer (ish) time this year, when CPU performance started to drop.
I just thought the PC was getting old, but thank you to all who posted on here, as using a desktop fan blowing air at the pc gradually cooled it down and CPU performance went back to normal again.
I think it seemed to occur when Intel had that security issue on their chips this summer time. After the quick roll out to solve the security floor, my SB1 performance started to drop. And it's never resolved itself, until using the fan!
So I'm no pro, but I think it's an issue somewhere in the coding, rather than hardware. And given it isn't Surface Book 2 exclusive, but also SB1, then maybe another indication not hardware issues, but coding.
Anyway, thanks again to all for solving my issue for me. I'm currently holding out for the Microsoft event on 2/10/18 and hope there's something cool, as I will be upgrading before Christmas. For some odd reason, I'm having faith microsoft will identify
and solve this issue, hopefully in the short to mid term.
Cheers