If you are running couple of sites on a small VPS and you have 512-1024MB RAM, you are probably not amazed by mysql eating 400 just to start.
You can cut 200-300 of that memory usage by disabling performance schema in your config file – usually called my.cnf

performance_schema = off

If you are wondering if you should disable it and what will happen – you most probably don’t need that feature so it is safe to disable it. In a few words – it is to help you tune the SQL server, queries, find bottlenecks etc.
You can get more info here – MySQL performance schema