My Mac is very laggy

Hello
My IMac is very laggy when i play tanki online even when I have everthing closed. It is not an old one but also not the total newest one. I wanted to ask how i can fix that or what ur ideas would be. One thing is that i also tried with minimizing the play screen but there it laggs even more. 
Please help me because i love this game but right now it is difficult!
Thx
sesese8627