Does anyone know what is the SIM indicator?

When enabling the simple mode of the Performance Overlay, among the four indicators displayed in the upper right corner, the game experiences noticeable lag when the SIM value reaches 60, while performance becomes smoother when it drops to 30. This issue was particularly severe when the game was first released. Back then, I didn’t know what setting I changed, but it suddenly dropped back to 30 on its own, and the game became as smooth as it was during the beta test. However, after the recent winter update, the problem has become even more severe, and now I simply can’t get the SIM value to change from 60 back to 30 no matter what I try. Does anyone know the meaning of the SIM indicator and how to adjust or optimize it? 😭