performance issues

i'm having some issues on the performance of my galera cluster, i think it could be because i need to optimize my.cnf configurations...

i currently have a php application with a database that has more than 1000+ tables and some of those tables have between 2 and 8 millons rows.

i installed your galera cluster on a virtualized enviroment (Openvz) on a dedicated machine Xeon E3-1245 V2 @ 3.40GHz with 32gb RAM.

i have the default 3+1 nodes on virtual machines with 4gb of ram each.

i have default my.cnf except for table_open_cache wich im increasing on a regular basis...

i would like some tips about optimizing my setup because i have very slow queries when working with the big tables.

i can provide any information needed or access to clustercontrol, many thanks.