Odd, I’m not experiencing that to any extent, nor did I ever have it when playing on Steam. Does this happen when playing the base game on GOG/itch too? Does it happen in all scaling modes? (try also: true nearest neighbor scaling which the game’s settings don’t actually offer, as it pixelates rotation effects like those charged wave VRP teleportation elements from Gaia’s Garden) What graphics hardware do you have? Sounds like it could be an integrated graphics issue or something, at least that’s the kind of thing I know to be a fairly common cause when miscellaneous rendering/performance issues arise that don’t affect others at all.
Strangely, I never had this issue until well into the New Home DLC, during what I think is the final stretch of the dungeon. That might be the only place it happens; I'll have to check that.
Don't remember much about my PC's graphic specs off the top of my head, but I'm using NVIDIA and the computer's beefy enough to run MGSV without issue.
Turning down pixel size greatly reduced the lag's severity. It's still somewhat noticeable, but otherwise the game's quite playable, so I'll consider that my solution. I'd prefer not to rely on a 3rd party fix if I don't have to. Thanks so much for the help!