I used quite a few plugins and scripts today in relatively big projects (one was over 400 tracks) so I can't say for sure it's just vanilla REAPER either.
I think I just found an important clue by taking a spindump of the process:
https://pastebin.com/raw/wiG8AL66. Mac_GetProcessMemUse. And indeed, the timing of today's stutter is consistent with the refresh rate of the infobar and disabling "CPU/RAM use, time since last save" makes the UI smooth again!
I'll check if it's always the case over the next few days...