Not sure if anyone still plays this on PC, but this tweak boosted my FPS by about 10.
http://forums.eidosgames.com/showthread.php?t=136427&page=2
Note that this could cause some graphic corruption. Nothing harmful but there's a chance it might not work too great for some people, there's a reason they didn't use it as default. If you want to turn it off, just change the value to 0.
http://forums.eidosgames.com/showthread.php?t=136427&page=2
If anyone needs more detail about adding the new Registry key hopefully this will help.
Open regedit and navigate to HKEY_CURRENT_USER\Software\Crystal Dynamics\Tomb Raider\Graphics
Right Click anywhere in the empty space and select New > DWORD (32-bit) Value
Name the new value MultiCoreFlushMode then double click on it and change the value data to 1.
Note that this could cause some graphic corruption. Nothing harmful but there's a chance it might not work too great for some people, there's a reason they didn't use it as default. If you want to turn it off, just change the value to 0.
