What you are experiencing is not lag. It's just low framerate caused by trying to use graphics settings higher than what your computer can handle.
Go to your graphics options, and switch everything to 'off' or 'low', especially shadows and anti-aliasing, then see how it runs.
If your framerate seems ok, go back to the options and switch things on one at a time, going back to the game in between each change to see if the option you have changed has affected the framerate.
Changing the option to download hi-res skins (1024) to lo-res (512) can also have an effect, as the GPU won't need to load as much texture data into the Vram.
FYI:
Lag is the effect of a driver with high ping on an online server. They'll appear to 'teleport' from place to place, or they may appear to be swerving off the track, only to reappear on the track again every few seconds. This is because the position information sent from their computer over the internet is taking longer to update due to their connection being slowed by either their location relative to the server, or by another application running on their computer which is also trying to use their bandwidth. The server tries to approximate their position on the track, but if the driver adjusts their line or speed, the server won't update that info until it receives the new packet a few fractions of a second later.
hope this helps