Suddenly 5fps on the login screen, 3fps ingame

Yesterday everything was fine, today the game is unplayable.

Performance graph shows the problem is mostly the GPU, which is spending ~200ms on each frame. The options menu correctly shows my graphics card (RTX 4070), and nvidia-smi shows pathofexile.exe as a running application, so I guess it's using the right graphics card rather than integrated graphics. Reducing the performance settings (fewer lights, etc) does nothing. Changing renderers has a very slight effect (Vulkan gives me 6fps on the login screen instead of 5, for example). The laptop otherwise seems to work fine.

Already removed/reinstalled my nvidia drivers, no effect.

Any ideas for other diagnostics/fixes to try?

GPU-Z told me my graphics card was locked at 210Mhz, which apparently is what they do when they're having voltage problems or think they're overheating. Windows had just installed an update; I uninstalled it and rebooted, and the problem went away.

Was that really causal though? Who can say, computers are mysterious.