I have sometimes seen that some applications that are built in php are comparatively faster than those that are developed in .net or java. I have seen some other projects also behaving same. Generally application built in .net are comparatively slower. Best example I have seen is facebook. It works well even if there is low connection or low bandwitdh but at that time other sites dont work. Does the underlying technology really matter??
Given a comparable level of skill for the people developing / operating the site, comparable infrastructure, and enough development admin time to optimize the system to the abilities of those people, then yes, it makes a lot of difference.
However, IME, the biggest differentiator between languages is not what their intrinsic perfromance, but:
the level and availability of skilled developers / admins
the amount of effort required to achieve a given end result
The lower barriers of entry to PHP programming compared with Java, means it is a lot easier to find a competent software engineer proficient in Java compared with one proficient in PHP, despite PHP skills being much more widely asserted.
OTOH, if you look at software metrics, PHP almost always trounces Java in terms of LOC/productivity. And unless you are Google or Facebook, salaries are a much greater cost than hardware - if it's slow, buy more infrastructure.
Don't get me started on .net