This is not a bug but a suggestion. Its not exactly a how to either

I noticed that mysql has caching turned on, but is query_cache_size=0 which effectively turns it off.
(This seems to be the default Centos distribution settings).
I would suggest starting off with (in my.cnf), assuming that you have enough memory. (16Mbyte for various cache should not cause memory issues)
query_cache_size=16777216
tmp_table_size=16777216
max_heap_table_size=16777216
thread_cache_size=32
query_cache_limit=1048576
table_open_cache=128
You may or may not need to increase this, depending on your actual mail load.
I would suggest using these tools to figure out your requirements.
MysqlTuner.pl
https://raw.githubusercontent.com/major ... qltuner.pl
And mysql tuning primer.
https://launchpad.net/mysql-tuning-primer
Regards,
Michael