There is no right answer. Despite what others say, php is perfectly suited for a lot of needs. Runtime may be your issue, but you also have to think of supportability.

If your looking to optimize your site, as messenlinx suggested yslow is a good place to start. Next, I would suggest looking at your queries, ie a select statement across a big table vs. a more refined query across a smaller data set will do wonders regardless of the language.

If your data is updated infrequently, you can look at caching your queries in the data level. If not, maybe look at memcached?