I run the numbers and monitor New Relic charts on large WordPress sites all the time. Yes, some request time is spent doing I/O, but the majority of request time in any healthy PHP (or Ruby, Python, etc.) site is spent in PHP.

(When more time is spent elsewhere, you have a problem that needs to be fixed. For example, waiting for I/O because you've done something silly with a database query or you've pushed it beyond its scaling limits and everyone's waiting for I/O.)