I take a lot of screenshots to the clipboard and paste them into apps such as obsidian.

The larger the screenshot is the longer it takes and at a certain size (my display is 8k) there is a high probability of failure, as in there is nothing in the clipboard according to the clipboard manager in the xfce-panel.

For example taking a screenshot that covers 3/4 of my screen might take about 10 seconds on my ryzen 5700g (8 cores) with 64GB RAM. First it is single-core load but later all cores get busy.

Im running manjaro stable with the xfce-desktop and tried 3 different screenshot apps: xfce-screenshooter, gnome-screenshooter and shutter.
It's the same behaviour with all of them.

It seems to be the process of copying the screenshot to the clipboard that takes all this time and possibly fails. When i directly save or open the screenshot in any app it is instant and always works.

Has anyone ever experienced disappointment at the speed of which screenshots are copied to clipboard?

I always save screenshots to disk before use, so can't picture your use case. However, if you try your activity in some other DE you could narrow it down to whether or not XFCE is at fault. IceWM and TDE, where available, are among lighter weight DEs that don't require as much RAM or disk space as higher profile DEs, and shouldn't interfere with your XFCE configs.

Same goes for your DM. If you normally launch from LightDM, GDM or SDDM, try with startx, XDM or TDM instead.