innodb_buffer_pool_instances is, perhaps, the single most important tuning parameter in MySQL 5.5. And the default of 1 severely caps MySQL performance on any recent hardware.
A more reasonable default is called for, e.g. (innodb_buffer_pool_size in GB + number of CPUs)/2.
This observation is coming from my recent experience of “first run” tuning of MySQL 5.5 on several installations: this is the first thing that was changed on all of them.