I am noticing that performing many "update_by_query" calls is very fast initially, handling several thousand a second. As time progresses though, this crawls to only a few per second. Looking at memory/CPU usage, it doesn't appear to be running up against any hardware bottlenecks, so this could be a settings issue.
Are there settings I can check that could lead to write requests being fast at first but then slowing to a crawl? Is there some "keep alive" that is holding a queue full, despite requests being completed? Anything like that?