AeroSpace does indeed suffer from a performance issue but this appears to be inherent to the available macOS APIs: https://github.com/nikitabobko/AeroSpace/issues/131
The way you reproduce it is open Factorio.app and quickly switch between workspaces while the game is loading.
FlashSpace seems to suffer from this problem as well based on a quick test.
With regards to configuration, I think it's cumbersome to configure shortcuts per workspace. In AeroSpace, every letter and number on the keyboard is automatically a workspace and shortcuts are global.
All that said, this is a fantastic tool and I would prefer a tool like this that addresses the pain of macOS workspaces as its primary goal rather than being part of a tiling window manager.
EDIT: After more testing it appears that FlashSpace does not support having different windows of the same app on different workspaces, this is a dealbreaker for my workflow :(