can't you use a comet connection with something like orbited instead of polling? i wouldn't trust polling via HTTP GET with "real-time accurately synchronized countdown" -- a couple of small delays can skew your entire countdown, and delays are easy to come by across the internet. especially with creating multiple connections.

as much as i dislike like it, i doubt the problem has much of anything to do with php. a simple fastcgi server hooked into lighttpd would have probably had the same outcome of better performance, or even apache with mod_php.