Nightreign runs with an unusually low frame rate on Linux, while CPU and GPU usage both remain very low. After searching around on Google, I found the answer in a GitHub Proton issue:
https://github.com/ValveSoftware/Proton/issues/8751#issuecomment-2957426376
https://www.phoronix.com/news/Linux-Splitlock-Hurts-Gaming
Some games, anti-cheat systems, drivers, or runtimes can trigger split locks. When Linux detects this, it throttles the related process to prevent it from slowing down the whole system.
This mechanism is good for system fairness, but for games it can show up as lower frame rates, stuttering, slower loading, or increased input latency.
After disabling it with kernel.split_lock_mitigate=0, the kernel no longer penalizes the process that triggers split locks, so game performance can recover. The tradeoff is that if a program continues triggering many split locks, overall system responsiveness may be affected.
For a daily-use Linux PC, I think it is completely reasonable to disable this and it should not have much impact.
The specific fix is as follows. To apply it once and take effect immediately:
sudo sysctl kernel.split_lock_mitigate=0
To restore the original behavior, set it back to 1:
sudo sysctl kernel.split_lock_mitigate=1
To make it persistent, add the following to /etc/sysctl.d/99-splitlock.conf:
kernel.split_lock_mitigate=0