Stylesheet; consider removing top level box shadow in .solid-csd
The inner box shadow applied on .solid-csd windows is barely noticeable but has a huge impact on performance with the GL renderer. People who have non-functional GPUs are generally also people who don't use a compositor or a sane window manager and thus get .solid-csd. This makes the difference between the GL renderer being usable on llvmpipe and unusable.
On my testing on a 1440p dual core completely software renderered system using llvmpipe, there is generally an over 6 fold increase in frame-rate after removing this single box shadow in Gtk Inspector.
Although the default renderer with llvmpipe is still generally slower than the cairo renderer, without this box shadow GNOME Builder increases from 5 to 30 fps, an empty window with a button goes from 7 to 60 fps, frames demo (with Default theme) goes from ~10 to 60 fps, Libadwaita demo in most cases goes from 6 fps to 60 (but not quite on the Avatars demo).
Using a simple border instead of a box shadow on the window contents reintroduces the performance issue.
Here is a comparison of the appearance with and without the box-shadow.
Before the shadow made the resize border almost the same color as headerbar, etc. Now it has the window background color, changing the window background color would also break some views.