No, I'm sorry. I only know what the documentation says, that ES uses heuristics to trigger flushes and that the user can trigger this manually in order to reduce the recovery time when restarting nodes.
But are you sure flushing is the cause for the slowness you experience every few seconds? If you've turned off refreshing it could still be the garbage collector kicking in. Perhaps you could check that?
I regularly re-index hundreds of millions of documents and terabytes of data and the Reindex API uses bulk indexing underneath, but I've never experienced the problem you mention. All I ever do is turn off the index refreshing and shard replication, after that it's all smooth sailing.
I hope you can figure it out or that someone else, who knows more about the flush mechanism, can help you.