Q: safari laging

Hello patrykfromlondon,

I understand a game you play on the Internet with Safari was working fine but in the last few days it started running slow with lagging performance.

Do you remember if there were any changes when it started being slow? 

Unsupported third-party add-ons may cause Safari to unexpectedly quit or have performance issues - Apple Support

If it's a Flash based game insure you have the latest version.

Adobe Flash Player updates available for OS X on March 20, 2015 - Apple Support

If not we still need to isolate the issue.  You may want to start with the isolation steps here.

Isolating issues in Mac OS X - Apple Support

Then maybe focus on Safari itself.

Safari 8 (Yosemite): Safari troubleshooting

Thank you for visiting Apple Support Communities.

Nubz