Linux

I would start by checking for any sort of errors in your system logs, such as /var/log/syslog or using dmesg -w. In my experience, Linux is almost universally faster than Windows.

Since you mentioned high cpu temps, do you have a water cooler? It’s possible that the pump is running at a reduced speed or not at all. If it was functioning fine on windows that leads me to believe that it’s not hardware related like some are suggesting.

With temps that high in Linux and Windows, it almost sounds like the AIO water block is falling off the CPU.

This sounds like a hardware problem. Keep in mind that thermal paste doesn't last forever. I'd rebuild it.

I agree it sounds like your cooler is failing or not installed correctly.

Based on your update, are the AMD drivers loaded and working? Maybe it's using CPU for rendering instead of GPU.

Are you using snaps? Snaps are notorious for God awful performance.

Do you use some weird cooling solutions? Drivers may be an issue. The other possibilities I can think of are hardware related.

You didn't mention what kind of cpu cooler (that I see). Have you checked there is fan spin for cpu fan? Possibly in windows you've got software controlling fan speed and that link is missing in Linux so it's trying to just passively cool it? Even more complicated to diagnose if it's an aio but could be similar with no pump running.

What i would figure out first is why tf your hardware runs on max all the time.(Maybe a bugged out program?) You can do so via top/htop/btop etc.

Generally I would say the exact opposite is the norm. Every Linux flavor I have tried feels a lot faster than any of its peer windows versions

Honestly it sounds like a slight hardware issue that was made worse by Linux not having hardware modules written by the manufacture

I'm not sure about how much up to date Kubuntu's drivers are but maybe, if the software is at fault, changing to a more bleeding edge distro, like Fedora KDE, would solve this issue with temperature.

Lubuntu or xubuntu are quicker (especially on lesser machines), but it does sound like you've got cooling issues.

I always find Kde heavy-handed with resources to deliver the GUI.

The thing I experienced on my laptop was: I used on Linux Max Performance while on windows I let it be the default (balanced or smth I think). The result: my Laptop hit way too often 100°C when playing games that my CPU throttled to 800mhz. It was a quick fix by just using balanced instead so it can decide for itself when to cool a tiny bit to not throttle, like windows.

There are multiple tools to set the Intel Power Management Profile to "Balanced" instead of "Performance"

When you know it's overheating, you could as well try and remove dust, with some compressed air. It can't hurt.

Edit: Actually there's plenty other that thinks this could be a combo of software and hardware, so my point is still valid.

Check your hardware if you start getting overheating problems.

Did you change some hardware when you switched? Put a new SSD in? Maybe you knocked something for cooling lose. Or blocked some vent.

Do you have the Windows installation still? You could try to see if it's showing the same symptoms. Or download some other Linux and boot that from USB.

This is purely based on my experience but somehow I think that Pop OS works better on laptops. System76 is a laptop manufacturer so it makes sense they'd optimize for that, I suppose. You may want to give that a try

Some distros have always seemed to perform better than others for me. Ubuntu was always lagging, mint has always performed very well. I never figured out why. But some hopping might be the easiest solution.

That's really interesting that's for sure, since I'm experiencing the opposite of it? Did you try Wayland perhaps? It handles displaying stuff in a different way. You can choose it in the login screen on the bottom left side probably.