Chrome does this here:
https://source.chromium.org/chromium/chromium/src/+/master:content/gpu/gpu_main.cc;l=252;drc=d8c3f0c1b184ffb124792c1a5b7c00e243c453e3

I find that code in Chrome surprising because IIRC normal Windows applications aren't allowed to use anything above NORMAL_PRIORITY_CLASS without special permissions but maybe things have changed (or I may mis-remember).

Implementing this should be relatively simple, it's just a matter of modifying ParticularProcessPriorityManager::ComputePriority() to use PROCESS_PRIORITY_FOREGROUND_HIGH and then modify hal_impl::SetProcessPriority() to use ABOVE_NORMAL_PRIORITY_CLASS for that level.

There is one important thing to keep in mind though: if no special permissions are required and this works you have to double-check that the higher priority doesn't affect the rest of the program. We did implement this in Firefox OS back in the day and it provided butter-smooth scrolling and perfect 60Hz refresh but significantly starved the main process' main thread dragging down the whole system. It was different hardware (low-end dual-core Android SoCs) so what applied then might not apply anymore but it's worth keeping it in mind.

See also the summary from https://phabricator.services.mozilla.com/D143056, which has some caveats to increase the priority of GPU process ("we have too many gfx threads").

https://hg.mozilla.org/mozilla-central/rev/18f23fa58771