Leaving this up in case anyone else ends up in the same spot I was, but, I don’t know if KWin performance measurement would have helped anyway, as my specific problem was because the Nvidia 555 drivers have enabled by default a feature (GSP offload?) that’s apparently still buggy in some KDE setups.
My other desktop was too old to have that feature even physically possible, I think 
Relevant links:
Folks discussing a resolution to this issue: https://www.reddit.com/r/linux_gaming/comments/1d0mqmz/nvidia_555_beta_gsp_firmware/
Coming work that should hopefully help without workarounds: https://www.reddit.com/r/kde/comments/1eahyr5/nvidia_users_5602803_beta_aims_to_fix_frame/