I have a suggestion - turn off raytracing and shaders. After this, mess around with the other settings to see what might be causing that lag. Then try it with shaders back on, and mess around with the settings until you can find something comfortable. (Raytracing will cause definite lag if the blood hitting the floor will lag you)
As Killa said, it is also because you might not have a 'powerful' enough computer to run the game on certain settings, so post your specs so we can evaluate it.