The client uses up to 100% of the CPU's resources. Is there something I can do?
Yes. Go to Options->Graphics and check out your frame rate limit. Try to lower the limit to a decent value. It may be necessary to make some experiments here. Once you have finished doing this your computer's CPU should use less of its resources on Tibia.
The effect can also be caused by malware/adware/PUP constantly reloading into the game. Please scan your entire system with anti-malware software such as Malwarebytes Anti-Malware to remove it.
If this does not help, please try to deactivate all other programs that are running in the background on your computer one by one and check if the situation improves. It is possible that a program that is not detected as malware is trying to attach itself to the Tibia client and is being blocked by BattlEye.