A small issue that I recently addressed with the tool I shared:
when your proxy file has different resolution than the original file, it causes problems with some filters. Specifically the ones that deal with height and width, like crop or size/position.
What I ended up doing is I use proxy files of same resolution as the original clip, but a quality that is low enough to introduce boost of speed into UI responsiveness, yet high enough to still look good in the preview window.