Paul Reinheimer had a problem - when he was making asynchronous requests back to his server from his frontend (Ajax) there was a slowness he noticed when more than one connection was fired off. In this new post to his site he traces through how he found the answer and what he did to fix it.
Digging a little deeper into the queries being executed, I was expecting return times in the order of 200ms, not the several seconds I was seeing. Installing XHGui only furthered my confusion: session_start() was the culprit with incredibly high run times.
He thought first about the number of session files (stored locally) being too large and causing issues, but that turned out to be a false lead. Instead, the issue was something PHP does by default...and does correctly. When PHP executes, it locks the session file, preventing another process from writing to it. This caused the delay he saw until it was unlocked. His solution? Use session_write_close immediately after writing information to unlock the session for further use.