Well I'm not referring to reading/writing from a data store, but actually processing the data in registers and such. Arithmetic operations and that sort of thing.
High-density ARM servers are not ideal for every application. However, there are plenty of applications where you don't need massive amounts of computational grunt in each CPU, such as web serving, hadoop, mail and DNS servers, etc. Also, the density and lower power/cooling requirements can often offset the lack of performance. Each slab has a 260W PSU. The whole system can't pull more than 260W, ever. Some Intel CPUs need 200W just on their own.