P: Mixer brush lag at different zoom levels

I notice an issue using the mixer brush with sample all layers on, the performance is worse at certain zoom levels and at times it would appear to gradually decrease for every stroke until reset by changing the zoom level.

I think this only happens on some zoom levels for example in my current res document the worst performance is at 50% or less

here's what I do say at a zoom level of 50% : 

- select a brush preset from the the tool presets, 
-sample a part of the image and do a series of strokes of about the same length, 

when I zoom in to say 66% there is a noticeable performance increase in doing the same strokes. they finish faster and its more fluid. 
also it seems if I'm at 50% (slow) if I zoom out and zoom back in, the first few strokes appear to be a bit faster and then it gradually slows down. 

this is affecting my workflow as I find myself having to zoom to reset the performance of the tool otherwise it grinds down to a crawl. 
I'm on windows, latest PS 

my brush settings are: autoload and autoclean off, some wetness (30%), 100% load, 0%mix 100% flow. I have a noise layer set to overlay on top of the stack which the mixer brush samples with sample all layers on. 

is this expected behaviour? can anyone reproduce? and any settings I can look at to minimize the impact?