There are many reasons:
1) What "db" are you using and the performance characteristics of that engine.
2) Resource/object contention between your threads/tasks
3) The amount of data being saved
As a side note my
RaptorDB - The Key Value Store V2[
^] can handle 400,000 writes/sec on an i7 system, so that is some thing to think about if you really need high throughput writes.