Use Lanczos filtering for downscaling fractionally-scaled clients that don't have their owen native fractional scaling support
With the new Wayland protocol and Qt implementation to support fractional scaling on Wayland, KDE and other Qt apps should look better and be more performant when fractionally scaled.
However not all clients will have support for this, notably GTK apps. As a result they will not benefit, which can be noticeable since most of us use GTK-UI web browsers.
We can't fix the performance issue, but we could fix most of the visual issues by using a Lanczos algorithm to downscale them. This should reap significant wins in the visual department at least.
Because it would be more computationally intensive than linear or bilinear filtering, it might be worth exposing this as a user-facing choice somewhere, offering a choice between better appearance and better performance.