apparently, GTK_RELIEF_NORMAL is causing noticeably higher CPU usage than GTK_RELIEF_NONE

Steps to reproduce

If someone could whip up a small gtk sample program to showcase/test whether painting/refreshing a bunch of GTK_RELIEF_NORMAL buttons vs GTK_RELIEF_NONE sees any increase in CPU performance (ie. $ top -d 2) that would be pretty amazing! (as I've no idea how to do that, I've never done anything gtk-related)

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Current behavior

I reported an issue here about Window Buttons plugin of xfce4-panel causing noticeably higher CPU usage(depending on the number of buttons ie. 15, can be 25-39% in each of Xorg and xfce4-panel processes) when its Show flat buttons setting was disabled(aka it was setting gtk_button_set_relief() GTK_RELIEF_NORMAL for all tasklist buttons - all those buttons on the taskbar that represent open windows) than the basically no increase in CPU usage(really) when Show flat buttons setting was enabled(aka GTK_RELIEF_NONE).

As it seems to be a gtk3 issue, I'm reporting it here, because xfce4-panel with its tasklist plugin(aka Window Buttons) doesn't seem to do anything else that would appear to cause the increase in CPU usage.

Expected outcome

CPU usage should not increase when buttons are GTK_RELIEF_NORMAL(compared to when they are GTK_RELIEF_NONE), or if it does increase it shouldn't more than 2%.

Version information

local/gtk3 1:3.24.10-1 ArchLinux x64

  CFLAGS+=" -DG_ENABLE_DEBUG -DG_DISABLE_CAST_CHECKS"
  arch-meson gtk build \
    -D broadway_backend=true \
    -D colord=yes \
    -D gtk_doc=true \
    -D man=true

Additional information