I'll keep it short. This simple fix has brought me from 45fps in big fights and 20fps on certain maps all the way to 60 fps in all circumstances.

Once the game is running, go to task manager, switch to "details" tab, right click on "Last Epoch.exe", select "Set Affinity" and uncheck "cpu 0".

Detailed instructions:
1) go to your Task Manager in windows
2) switch to "details" tab
3) right click on "Last Epoch.exe"
4) select "Set Affinity" and uncheck "cpu 0"
5) click "ok"
6)???
7) profit

You can automate this process with tool called "Process Lasso", by running the game and then in Process Lasso, right clocking Last Epoch.exe, sellecting "CPU affinity" and then unselecting CPU 0. You can even deselect CPU 1, might help further, but I have not tested that.

As long as Process Lasso is running, every time you launch Last epoch now, those selected threads will not be used for Last Epoch