And our influx setup does bursts of 500k writes per second with a lot less operational overhead than Cassandra. For time series data, a general purpose database is always going to be slower for both reads and writes. The data on disk and in memory is simply laid out differently.

For an excellent academic example, see this paper of Facebook's gorilla in memory TSDB:

http://www.vldb.org/pvldb/vol8/p1816-teller.pdf