I installed a heavy plugin and my site is runnin low

Hello WPMUdev, i have a problem.

I installed a quiz plugin and when i started to use it (just making some configurations) my site started not working fine.

I am having trouble when loading it, sometimes it wont even load and sometimes it loads with CSS errors. It still loads fine 1 out of 10 times.

Then i deleted the plugin with all of its files (following wp procedures) but i am still having this issues.

I checked another site i host on the same server and it is having the same troubles.