kumomta/docs/reference/kumo/define_spool/flush.md

flush

Whether to flush data to storage after each write. The default is false. When set to true, a backend specific means of flushing OS buffers to storage will be used after each write to encourage maximum durability of writes.

Setting flush=true can be incredibly harmful to throughput, and, depending on your local storage device and filesystem selection, may not meaningfully increase durability.

kumo.on('init', function()
  kumo.define_spool {
    -- ..
    flush = false,
  }
end)