It looks like the Slice Outline effect is heavy on resources.
I checked with two strong computers.
4K composition + Slice Outline source using only 3 slices (polygons) and frames start to drop.
the same with regular slices - more then 10 slices and frames start to drop dramatically.
With easy outliner demo from juicebar its even worse.
On the other hand the "Slice Transform" is super fast! 60 polygons and still 60fps