If you are using Windows 7 64-bit Ultimate and see screen flashes, pauses, or hiccups each time you press a key, try turning off the Windows event log. To disable the Windows Error Log service, click on the Start orb in the lower left of your screen and type MSCONFIG into the search bar. Open the utility, select 'Services' and then untick the 'Windows Event Log' entry. After clicking 'OK' you will be prompted to reboot so the changes can take effect. After a reboot please try the game again.
On certain systems, it may help to make sure that the "Mipmap Detail" slider in the AMD Catalyst Control Center>3D Application Settings is set to Quality and not Performance.
If your setup is within spec and you are still experiencing slowdown, lag, or frame-rate issues:
Run the LANLauncher.exe.
Enter "-str" and/or "-npd" and/or "+jobs_numThreads 0" (without quotes, just the dash and the text with a space in between each command) in the command line field for single threaded renderer, which may improve performance or compatibility on some systems
Finally, change your DirectX setting in the graphics option of the launcher and try both DirectX 9 and DirectX 11. Depending on your operating system and graphics card, one of these options may result in better performance than the other.