Hm, possible solutions:
Can you try going to the settings menu via main menu and experiment with the settings there? The resolution/ fullscreen /quality settings?
If it doesn't help, can you provide the following information?
1. Does this happen with other games?
2. Can you press CTRL + Escape and check in the "Performance" tab, if either , CPU, MEMORY or GPU are capped to 100%?
3. What OS do you use? (Windows /android / etc...)
Let's get this fixed together!