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!