Also on Niri with fractional scaling and pgtk. IIRC the issue here is that for programs that don't support wp_fractional_scale, compositors deal with this by rendering at the next highest integer factor, and downscaling it down to achieve the desired factor. This is much, much worse for Emacs since it uses GTK3 and doesn't support hardware acceleration with Cairo, at least according to Po Lu [1] so you're effectively rendering via CPU at a very high resolution.

For XWayland apps Niri just renders at the native resolution (ignoring scaling), meaning a decent workaround is to use the Lucid/non-pgtk version and manually scaling up the UI. Unfortunately I go from scaling at 1.25 on my screen to 1.00 on my external monitors, which means I can't use the non-Wayland versions without messing up the font size on either my desktop and monitor.

[1] https://mail.gnu.org/archive/html/bug-gnu-emacs/2024-09/msg0...