Performance issues and loud fan

my Xbox One X has some performance issues maybe you have an idea what it is.

I mentioned when I play some games with heavy load (eg Anthem, is my test case here) that the fps are getting low and the fan is blowing really hard.
So I thought it should be an thermal issue and I replaced the thermal paste and cleaned the console (there was nearly no dust in there). Now the console is really quiet in menu and in games with low performance requirements. But when the console needs to use the whole power the issue is there again. Now I tried to replace the thermal pads on memory but this does not change anything.

I have a second One X here (it is really new) without those problems.

It seems to be an thermal issue because the fan is blowing so hard. For me the only other sensefull thing is the cooler itself. Maybe the vapor chamber is not working correctly anymore or could it be a fan problem?

Would be nice if you have an idea.

Greetings from germany.