Hi,
It's been known for a while that rendering when zooming in on an area with filters is slow. This is due to Inkscape breaking the canvas into tiles and rendering each tile separately. Filter effects are recalculated for each tile that they touch. A typical canvas for maximized Inkscape window on a 1080p monitor is broken into about 25 tiles. If you are zoomed in so that the filter extends over the entire visible canvas, you are recalculating the filter effect 25 times.
I've added a preference under the "Rendering" tab to change the relative tile size (Rendering tile multiplier). Please play with changing the value for your most complex drawings. I am most interested in cases where a large multiplier has a negative impact. If there are no cases with a negative impact, I'll set the default value higher (it is currently 1).
Tav