xdg-document-portal is used by flatpak and snap apps to allow a program to elevate privileges outside of their sandbox (opening and saving files for example) and it uses a systemd call to achieve this.
This explains why those two processes are using high cpu cycles, but you’ll have to dig deeper to find the actual program that is causing it.
You will know what you have installed that might cause it, the other thing to check is it set correctly for your desktop (gnome, KDE etc);