Changing your Graphics Engine
Some people experience lag due to graphical reasons. Lowering your framerate might help, but changing your engine might, too.
For this, go to Options → Graphics → Advanced. Under Graphics Engine, you should see three buttons (DirectX5, OpenGL and DirectX9). Most people should have one of the latter selected. Select "DirectX5" and then OK your way out of there.
A way to skip these steps is to simply check your desktop shortcut's Properties (and if you don't have one, create it).
There, in Target, at the end of the line add a space, then engine 0, engine 1, or engine 2 to start up Tibia using DirectX5, OpenGL, or DirectX9, respectively.
You can find these and other tips on the Framerate page.