[PERFORMANCE] 30 FPS issue since Windows 10 Fall Creator's Update

I updated to the Fall Creator's Update today. Ever since then, when I spectate or play a match, I get a stable 60 FPS for a short period of time (~10-15s?) followed a period of a constant 30 FPS (not once over 30 FPS until it hits 60 again). Ping is fine. I've never had an issue with FPS before. The FPS limit is capped at 60.

Updating the driver does not resolve the issue. Lowering the display settings does not resolve this issue. System CPU is round 37-40% while at 60 FPS, and dips to the low 30s / high 20s while at 30 FPS. Memory usage is constant around 30%. GPU usage is in the 20-25% the entire time in both cases.

Display Settings: Very High Resolution: 2560x1440 Windowed Mode: Borderless Video Card: AMD Radeon R9 390 Series, driver version 22.19.677.257 (9/22/2017)

This issue is reproducible (for me) across reboots, matches, etc. I ran only the League client and game after a full reboot with no change as well.