mod_fcgid is more efficient for some classes of problem than mod_php...though not "faster" by a huge amount, it does require a lot fewer resources including having a lower process count for the same workload. Switching to mod_fcgid for all PHP on your site would mean you could remove mod_php from your Apache process, which is a big memory savings.

eaccelerator can speed up some PHP applications.

Profiling your code and removing the bottlenecks would be another good option.

Since PHP is historically not thread safe you probably can't switch to a lower process Apache concurrency model (though there seems to be the possibility to use a limited subset of PHP 5 in such an environment, I'd be hesitant to do so in production, and you'll spend an awful lot of time digging up the facts on whether the modules you're using are thread safe).

While I've seen suggestions of running a lighter webserver for assets that don't need the full execution environment of Apache...I would instead suggest you make a lean VirtualHost section just for those assets. Yes, it will still be spawning Apache processes, but Apache can share a huge amount of resources with all the other Apache processes. It's hard to see in "top" and the like, because shared resource usage isn't very clearly dilineated, but overall resource usage of a single Apache installation (with many processes) is probably going to be less than that of Apache plus a second web server, particularly if you've already cleaned up your Apache and gotten rid of all of the unnecessary modules. Only real data can tell us for sure, but I'd be suspicious of any advice that seems certain that adding another webserver to the mix is the best solution, without having data about your particular deployment.