Linux way way slower than Windows?

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.

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

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.

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.

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

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

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

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.

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.

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

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.

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

if your CPU is at 100%, running "top" will show you what's taking CPU time

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.

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.

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"

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.

One more thing to add. Snaps? I heard it takes pretty long to open.

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.

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

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.

Sometimes one might have issues with delayed opening of GTK apps, but I’m pretty sure there can be multiple causes. All misconfiguration, so it would be weird since you’re on Kubuntu.

@Canadian_Cabinet Did you put Linux on a slow hard drive? KUbuntu can be slow on poor speed hdds just like any other operating system.

Go to terminal and type in top .. Maybe something like fs-miner is chewing CPU cycles? File indexing can sometimes do this.

Open your activity monitor (or hit Ctrl+Esc) and sort by CPU usage. What's your computer doing?

Also, have you installed all necessary drivers? I've never had your problem, so there's a chance you don't have the right drivers.

Could you follow the instructions on https://linux-hardware.org/?view=howto , upload a hardware probe and share the link here?

While I doubt this will work, uninstall something like xdg-portal-gnome if you have it. It significantly increased app startup times on my machine