[merb]

> Such as code that hits the database a lot or relies on network data.

well the first problem is not a problem since you can cheat it. if you rely on the native database you can actually fake your driver and insert transactions (subtransactions or in postgresql savepoints) so for the whole test suite you just rollback to the latest savepoint, this saves a lot of time. (it's actually not that hard in java+di or in languages where you can monkey patc code.