Depending on which version of Call of Duty: Modern Warfare 3 you are playing—the original 2011 release or the 2023 reboot—the "45 FPS" issue usually stems from either a hard-coded engine cap or incorrect display settings. For MW3 (2011 Version)
Additional tips
Steps:
"If MW3 is stuck at 45 FPS: set com_maxfps 0, disable Fullscreen Optimizations, and cap with RTSS instead of in-game. For persistent lock, try DXVK (single-player only)." cod mw3 45 fps fix
The Shader Reset: