This might not be Xfce-specific, but it applies to it for sure. In the appearance settings, on the style tab, selecting “Breeze” or “Breeze-Dark” runs the memory usage from ~600mb on idle to ~1200mb. I was able to recreate it several times, to make sure it wasn’t just something else happening in the background. Only way I’ve found to “correct” it was select another and then reboot (rebooting with them still selected would keep the high usage even after the reboot). Not really an issue, exactly, and not looking for a “solution”, just something to be aware of.
I did not know, good data!
Themes in Xfce can cause havoc.
Several GTK3 themes can cause the CPU to permanently lock at 100%. Just as an example.
I’ve noticed that while GTK themes are gonna take more RAM, Xfce in particular struggles with this. I actually filed a bug report for this in Xfce’s gitlab after asking on their forums and being told to, because no matter what distro base I use, Xfce balloons super heavily in resources once themed. You’re not alone in this.