As it is said in the recent article "Google: Taming the Long Latency Tail - When More Machines Equals Worse Results" , latency variability has greater impact in larger scale clusters where a typical request is composed of multiple distributed/parallel requests. The overall response time dramatically decreases if latency of each request is not consistent and low. 

In dynamically scalable partitioned storage systems, whether it is a NoSQL database, filesystem or in-memory data grid, changes in the cluster (adding or removing a node) can lead to big data moves in the network to re-balance the cluster. Re-balancing will be needed for both primary and backup data on those nodes. If a node crashes for example, dead node’s data has to be re-owned (become primary) by other node(s) and also its backup has to be taken immediately to be fail-safe again. Shuffling MBs of data around has a negative effect in the cluster as it consumes your valuable resources such as network, CPU and RAM. It might also lead to higher latency of your operations during that period.