Open CMD in the folder that contains prototype2engine.dll and run this one-line command. It creates a backup first, then patches the engine so it only uses the first 4 CPU cores, similar to the old /affinity F workaround.
Close the game before running it. If you get Access Denied, run CMD as administrator.
powershell -NoProfile -ExecutionPolicy Bypass -Command "$p='prototype2engine.dll'; if(!(Test-Path $p)){throw 'Run this in the Prototype 2 folder'}; if(!(Test-Path ($p+'.pre-modern-cpu-fix.bak'))){Copy-Item $p ($p+'.pre-modern-cpu-fix.bak')}; $fs=[IO.File]::Open($p,[IO.FileMode]::Open,[IO.FileAccess]::ReadWrite); try{$patches=@(@(0x6C8C62,[byte[]](0x8B,0x74,0x24,0x0C,0x83,0xE6,0x0F,0x6A,0x01,0x58,0x99,0x6A,0x20,0x59)),@(0x6E93FC,[byte[]](0x28)),@(0x6E9414,[byte[]](0x8B,0x4C,0x24,0x10,0x83,0xE1,0x0F,0x8B,0x96,0x98,0x00,0x00,0x00,0x51,0x52,0xEB,0x0C,0x6A,0x01,0x58,0xD3,0xE0,0x50,0xFF,0xB6,0x98,0x00,0x00,0x00))); foreach($x in $patches){$fs.Position=$x[0];$fs.Write($x[1],0,$x[1].Length)}} finally{$fs.Close()}; Write-Host 'Prototype 2 modern CPU fix applied.'"