That’s interesting kenmo, that’s the same size canvas I habitually work on too, but I’ve never felt the knife to be generally to slow. And by slow you do mean that it lags behind your pen stroke, right ?
It’s true with some combinations of knife settings the tool can lag slightly behind my pen on a 5000 x 3000 canvas, but I generally have to push the size to 400 or 500% to induce this, and for me, that is not a common size of blender I need to work with for long or often.
Though there are a couple of exceptions to that in the presets that ship with AR, Tiny Frost for example being the worst and starting to lag when above 300% on the Essential Canvas, blending paint from the default Oil Brush.
So maybe the original poster was referring to the Tiny Frost knife, maybe?

As for the whole general area of using other apps in the same painting, I say use whatever gets the job done the way you want it.
I often swop between AR and an other graphics apps for colour correcting, as for me, AR’s Adjust Colours panel has always been a little bit of an eccentric approach. Something like the HSL adjustment in Affinity Photo would be brilliant for me!