I recently reimplemented some of the caches within RestKit using a `NSMutableDictionary` guarded by a dispatch queue instead of a `NSCache`/`NSMutableDictionary` + `NSRecursiveLock`. The general idea is that you use a concurrent dispatch queue to provide concurrent read access and then use barriers to obtain an exclusive write lock to the resource. There were significant performance benefits from the change as I was able to shift the cache updates on a miss into the background. The dispatch queue approach also outperformed `NSCache` significantly under the workloads I was testing -- it appears that some of its internal calculations on add/remove can be quite costly if you are adding/removing rapidly.

Its also very flexible, as you can do `dispatch_sync` if you need a synchronous fetch of the resource or use callback blocks to go as async as possible.