1. Check and alter your graphics settings
This is quickest and easiest things you can start with to see if it helps. Go to Tibia Options -> Graphics. First of all, make sure that your frame rate limit is not set to the lowest possible value. If it is, raise it to a higher value. Also try switching between various graphics engine and see if one works better than another. Finally, switching the V-Sync off may help on some machines.
2. Make sure you don't have too many applications running in the background
It is common to keep a browser with multiple tabs opened. Modern browsers consume quite some memory (this depends on the opened websites too) and this adds up with every tab that you have opened in your browser. Try closing background applications that you don't use and there will be more free memory for the Tibia client to use. On Windows, you can verify what consumes most memory in the Task Manager.
3. Update your graphic card driver
Outdated drivers can impact your frame rate. It is recommended to keep your graphic card driver up-to-date for the best performance you can get.
4. You can also take a look at the recommendations on the official Tibia website