How many times do you need to run PK lookups per second? […] These are "SQL" overhead. It's obvious that performance drops were caused by mostly SQL layer, not by "InnoDB(storage)" layer. MySQL has to do a lot of things like below while memcached/NoSQL do not neeed to do. Parsing SQL statements Opening, locking tables Making SQL execution plans Unlocking, closing tables MySQL also has to do lots of concurrency controls.
via NoSQL databases