G-Sync on, responsive
G-Sync off, unresponsive
This issue only happens with an Nvidia GPU connected to a G-Sync capable monitor via DisplayPort, and with G-Sync turned on.
The only difference in the above videos is G-Sync being turned On/Off after restarting Roblox Studio; all other variables are controlled. Please look at the Diagnostics Bar in the bottom right of the Roblox Studio window to compare performance.
Some observations include an extreme dip in performance when in the script editor, but a slightly less severe lag in the viewport. All Roblox Studio windows are affected by this lag. Additionally, the window colors appear to flicker light/dark gray, but this is barely noticeable.
The same issue existed in an another report, but has been closed after an engine update patch. However, I assume this issue has been reintroduced in a later update. I have always used an HDMI without G-Sync, but only recently have I used a DP with G-Sync, so I can not say when this started being an issue.
EDIT
I realize that there already had been an older report which talks about the same issue. They have included a temporary fix, but the root cause still appears to be Studio.