Constant slow FPS decrease while the game isn't running

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My game used to run at constant 30-50 FPS before about two weeks ago, but now it drops continuously between launches: it will run at 28 FPS one day, then 20 the next, and 15 the one after, until it ends up running at 3-4 FPS at best, making even logging in a painful slog. Reinstalling the game brought it back to the previous FPS levels, but the decrease remained and it currently runs at 7 FPS.