Now, even if you have a great machine, you'll likely encounter lag still because of how the game is designed & ported over from Xbox 360 / PlayStation 3 (PS3). No worries. I am here to help (for a nominal fee of $5000 ;))
If you are experiencing lag, a low frame rate per second (FPS), stuttering / microstuttering, screen tearing, or just frame skipping, then check below. We shall optimize your game!
Solve Dark Souls Lag
I suggest trying the FIRST method listed here, as the success rate is very high for making your game performance increase.
1) Get the latest DSFix. Uncap the FPS and then set a custom cap to 30fps. This works wonders despite the fact you are capping the game back to 30. You should notice a big improvement.
2) Turn off blood (particle) and lower your resolution
3) Disable Anti-Aliasing (AA)
4) Try Windowed Mode instead of Full Screen
5) Install the latest NVIDIA or ATI or Intel drivers (these provide bug & glitch fixes, as well as improve framerate)
6) The newest Dark Souls patch also helps improve the fps (plus fixes the stereo downmix)
7) Close / exit all unnecessary programs and services
8) Don't connect online in the game for multiplayer (...this solution is awful, I would connect anyway)
9) Some TV's cause the appearance of lag / input lag if you don't put them in PC or Gaming mode - so put them in these modes! Works on both PC and Console versions!
10) Open ATI Catalyst Control Center (if you have an ATI card) and go to 3D Settings -> and disable Use application settings for everything, then move all sliders to the left. Be sure that VSYNC is also disabled!
And that's how you stop the Dark Souls game lag! So now you can enjoy playing Dark Souls in its full gaming bountifulness!
Note though that certain areas of the game are prone to lagging anyway (such as Blighttown).
Even with the game fixes, you may lag in SOME parts of the game regardless.
Dark Souls - Micro-Stuttering
Your lag may be something different altogether -- micro-stuttering. This can be caused by DRIVERS for your keyboard and mouse (Logitech is infamous for this). Additionally, if you plug in your game controller and start getting epic lag, then that's even more proof that it could be what's causing your problems. There's two things you can do here!
-- Disable / uninstall the drivers being used by your keyboard / mouse (if they are the offenders) or of your game controller (if it only happens when using a controller)
-- Use a different keyboard / mouse OR controller (but if it is the controller, then the same driver may attempt to use your controller here so the above may be necessary)
You can look at your devices & their driver info by going to Start -> Run -- devmgmt.msc -- This area lets you uninstall or disable/enable what's in-use.