Currently, KDE does not support fractional scaling per monitor so you should use the oldest trick in the book: adapt your resolution!
E.G for a 17" UHD display and a 24" FHD display, set both resolutions at FHD and Bam! Fractional scaling at a much higher FPS than fractional scaling can ever achieve!
KDE is the desktop for the tinkerers that know what they’re doing… whereas Gnome is so simple it can’t do half of what KDE does (but it allows fractional scaling because it’s simple) 