I use Affinity Designer and Photo and they are very fast even at huge sizes (I do art). But they don't listen to user feedback or suggestions much if at all. AD/P can use OpenGL Vulcan or Metal (I use Metal) and it's amazingly fast for most operations. I also use Artrage 5 and it's abysmally slow probably not optimized for any platform either, probably not much different from Inkscape (though mostly bitmap drawing).
There's no reason blur, in particular, must necessarily be slow. Everybody (and their phone) has a GPU that's more than fast enough to do that in real-time. I wonder if the (cross-platform) interfaces that Inkscape are using on macOS aren't hardware accelerated. Or if it's using some syscall which is cheap on some platforms and expensive on others.
I haven't done any testing but I agree that Inkscape feels much slower than Affinity Designer.
[1]: https://affinity.serif.com/en-us/designer/full-feature-list/ [2]: https://forum.affinity.serif.com/index.php?/topic/18079-what...