Also, I realized that a the current hashtable view clears a timeout and creates a new one every write. (the idea here is to write only when things stop changing, but clearing and creating half a million timeouts actually takes some time.

Adding:

if(timer) return
timer = setTimeout(function () {
  timer = null
  async.write(data)
})

means that index gets built in 2 seconds instead of 7! that's a huge improvement! I also suspect this change will effect flumeview-reduce too.

Do you have a PR for this? Would be interesting in testing.