If you are experiencing lag while playing Tibia, one of the reasons may be that your compter cannot "draw" the game fast enough. Lowering your framerate may help!
In Tibia, click on Options, select Graphics, and click on Advanced. There will be a horizontal bar where you can change the rate that it will try to "draw" the game.
Making this setting too high will slow down your computer, and can cause lag. Setting this too low will reduce the strain on the computer, but the display will not be very smooth, and may appear jumpy. Play around with it until you find a setting that works the best for your computer.
You can find more information and screenshots on the Framerate page.