In addition to the CPU throttling itself due to thermals like you said, you should also be aware that all Ubuntu's are replacing more and more traditional packages with snaps for an increasing number of applications.

Step 1 (by the system) of opening a snap application for the first time (since boot) is to extract the compressed snap image, which makes startup time significantly longer (like several seconds for something you would expect to be instant). Once the application is started performance should be the same as if the application had been installed as a traditional .deb package.

You should also consider adding flatpak support and flathub. Applications installed as Flatpaks generally integrate better in the desktop than snaps, and flathub has a large and growing selection of apps. The flathub website has a few command lines you can copy-paste to enable flatpak and flathub support, then apps from Flathub will show up in the Discover app store.

Personally I use Kubuntu and use both Flatpak and snap apps, but generally prefer flatpaks when they're available. And any software where I don't care about having a recent version I install as a traditional package because it's more lean