I'm not sure I would call Cassandra slow. If the schemas are done well, it can be quite good for time series. Obviously this depends on the type of time series you're writing/querying.

Our biggest goal was writes & uptime. We sometimes do over 150k writes/sec. We also needed it to be up and accepting writes even if one node goes down.

We regularly take nodes offline for updates/etc and cassandra never misses a beat.

We ~really~ wanted to use influxdb, but as a startup we couldn't justify the cost/benefit over Cassandra since we have 8 nodes for the DB. I just went to the influx site to try to find the pricing again and it seems to be hidden now :/

EDIT: As a PS, just remember every one of the influxdb benchmarks ( that I've come across ) are single node. Cassandra is meant to be horizontally scalable. Testing a single node Cassandra is like testing a racecar on your driveway...