Before diving into the fixes, it is important to understand why this happens. Unlike a standard low-FPS scenario caused by a GPU struggling to render polygons, the 45 FPS lock is usually artificial.
Some users reported that Bitdefender and other antivirus software caused an immediate 45 FPS lock. Adding the cod.exe as an exception or temporarily disabling the "Advanced Threat Defense" feature often restores full performance. cod mw3 45 fps fix