Do you know what process is causing the higher usage. Something I have noticed is that just having youtube page open in firefox causes kwin_wayland process to use higher cpu usage even if I never start the video and this after a fresh boot to eliminate other factors.
Edit: To clarify when I say higher cpu usage. I just mean that it basically goes from 0% total cpu usage to like 2.5% total cpu usage and stays there regardless of whether not anything is happening on the page. Not to say that it using like 70% of my cpu or something. Htop has it bouncing around between 30% to 50% of a single logical cpu (AKA thread).
Edit 2: After re-reading the post. I saw that you mentioned that firefox and mpv are specifically using higher resource usage. In that case how are you measuring this? I remember seeing a similar post a few days ago.
Edit 3: Found the other thread, but in this case the user was talking about ram usage KDE Plasma 6 Ram usage - #12 by dalto. The thing is though is that problems like this are difficult to solve because resource usage of an application doesn’t usually change much from desktop environment to desktop environment because they don’t manage or assign resources. That is the kernels job. This is why I thought that you were experiencing an issue with kwin because that is a kde process that can go wrong under certain circumstances.