100x less insert performance with Galera multi-master
I've hit the same issue. 
The first thing to do is to check the following variables:

select @@innodb_flush_log_at_trx_commit;

select @@sync_binlog;


Setting innodb_flush_log_at_trx_commit to 2 and sync_binlog to 0 will increase the performance dramatically.

The reason is in certification mechanism and the nature of multi master behaviour. Inserts will be very slow because in order to commit the transaction cluster will ensure that all logs are flushed at the time of commit at every node.

Please be sure that your caches fit RAM with the following query as well:

select ((@@thread_stack +@@binlog_cache_size +@@read_buffer_size + @@read_rnd_buffer_size +@@sort_buffer_size +@@join_buffer_size + @@global.net_buffer_length +@@global.query_prealloc_size + @@binlog_stmt_cache_size) * @@max_connections + (@@query_cache_size + @@innodb_buffer_pool_size + @@innodb_log_buffer_size + @@key_buffer_size))/(1024*1024);