If you’re using any scaling, then it’s not easily fixable on X11; mixed-DPI multi-screen setups aren’t supported and don’t work well in Plasma on X11. However if you’re running them both at their native resolutions and 100% scale, then there’s probably a bug in our code somewhere. I do see that you’re using Plasma 5.24, so there’s a chance any such bug was already fixed in the big multi-screen overhaul in Plasma 5.27, which you can get by upgrading to Kubuntu 23.04. In general I don’t recommend sticking to Kubuntu’s LTS releases.
But still, I’d recommend using Wayland if you have more than one screen. The multi-screen experience is much better overall. If you’re using an Intel iGPU, the performance issue you’re seeing is might be fixed with Plasma 5.27.7. If not, please do submit a bug report so we can investigate and fix it! …But only after upgrading to 5.27.