Be warned that depending on end of script to close the session will effectively serialize concurrent session requests. Concurrent background "data retrieval" (e.g. applications such as AJAX or amfphp/Flex) expecting to retrieve data in parallel can fall into this trap easily.
Holding the session_write_close until after an expensive operation is likewise problematic.
To minimize effects, call session_write_close (aka session_commit) as early as practical (e.g. without introducing race conditions) or otherwise avoid the serialization bottleneck.
