Lee we switched to using memcached for sessions and this helped, but still seeing blocking, though less time.

If we open two tabs, in the first page fire an ajax request that takes 20+ seconds to run, then in the second tab refresh, the page blocks loading in the second tab, but now instead of waiting the entire 20+ seconds for the first tab (ajax request) to finish, it only blocks around 8 seconds. See screenshot:

http://i.imgur.com/wcpKae4.png

It seems to be consistent, nearly 8 seconds of block every time. Any idea on this? What other fileio could be blocking?