Apologies for the vague initial report, I needed more time than I’d like to admit to pinpoint what exactly was causing the issue.
The frame time spikes when a tween resizes any persistent part welded to another. This only occurs, or becomes an issue, if enough other persistent parts are scattered around the world and will not occur in studio.
The issue clears up right after the tween finishes and will only occur while resizing; repositioning or other property changes seem to have no effect.
I’ve set up a repro place with ~27k persistent parts to demonstrate, but it will still occur with fewer
Video
bug report.rbxl (1.1 MB)