Absolutely turn on the query cache
Code:
query_cache_type=1
query_cache_limit=2M
query_cache_size=64M
and log long/slow queries
Code:
log_slow_queries=/var/log/mysql/slow-queries.log
long_query_time=3
The cache can speed up complex queries requiring multiple connections amazingly...I've one query that w/o the cache takes 35 secs (2.5million entries), w/ the cache is immediate.