Krita works much slower with high resolution images
I am sorry, but this bug has no information we can actually use. Hell, you couldn't even be bothered to write a title? Bigger images will slow down your computer yes, this is a physical limitation of the way the universe works. More things to do: more time needed. You can try to circumvent this by getting a faster computer. This report has nothing that indicates that this slowness could be caused by a bug, so therefore it is not a bugreport, and I will close it as invalid.