title: Cassandra performance issue with Akka cluster sharding
body: <p><strong>My application:</strong> </p>
  
  <ul>
  <li>akka cluster 2.5.x;</li>
  <li>akka sharding with evenly distributed function;</li>
  <li>cassandra plugin "com.typesafe.akka" %% "akka-persistence-cassandra"(default configuration);</li>
  <li>dedicated single-node Cassandra 3.3;</li>
  </ul>
  
  <p>My app simulates kind of financial app that acts on an account supporting the following operations: create account, debit/credit. If an entity actor gets such requests it will persist an event and return back the ack response to a client. 
  It created 2 types of tests: one is with in-memory and other is with Cassandra plugins.
  In-memory plugin - the app kept up till 20k requests per seconds.
  Cassandra plugin - only 4k requests per seconds and we got the exception: Cassandra driver exception: pool is busy.
  Let me ask my question what bottlenecks I met with Cassandra?
  Thanks.</p>