Basically all modern CPUs are designed to run hot, because they do opportunistic boosting. In performance mode, they tend to boost the clock frequency as high as they can until they hit thermal or power limits.
So if you are running in performance mode, a better heatsink won’t even necessarily run cooler or with lower fan speeds. Instead the CPU will use the extra thermal headroom to boost higher. So you’ll get the same temperatures, but better performance.
The Macbook Air doesn’t have fans, so it hits thermal limits faster and throttles down. So its performance is not as good as a Macbook Pro with the same chip for sustained loads.
If you want to run cooler / lower fan speeds, you want the CPU to boost less aggressively. In the extreme you can turn off Turbo Boost completely.
The Turbo Boost Max Technology 3.0 setting in the BIOS doesn’t turn off Turbo Boost. TBMT 3.0 is a very specific thing, it just lets the CPU boost the better cores higher than the others. (Due to manufacturing variance, some cores can run at higher clocks than others). I don’t think it makes a big difference whether you disable it.
There’s no way to turn off Turbo Boost completely in the BIOS as far as I know. You can turn it off in the OS though:
As far as I can tell, the Boot performance mode setting just determines what mode the CPU boots in. But as soon as the OS starts, it sets its own mode, so the Boot setting doesn’t do much at all. Basically it just changes what mode is running while you’re sitting in the BIOS and for the few seconds before the OS starts.
Do you have the same power profile set in KDE and gnome? It’s a slider on the battery tray icon in KDE. Not sure about gnome. If it’s the same profile I’m a bit surprised, I thought gnome is lighter than KDE.