Memsql, has two different storage option - Row and Columnar store. Row store is for in-memory and columnar for disk based.
If columnar storage can provide more performance on disk based why not use it for in-memory too ? ex: Hana which uses columnar storage for in-memory.
It depends on what you need that performance for. The MemSQL in-memory rowstore is for "transactional" workloads, eg, ingesting a large number of single inserts, high-speed counters, and traditional OLTP-type work. The on-disk columnstore is more for "analytical" workloads, on the data warehousing spectrum.