Indeed, the performance bottleneck of a webapp is often the database. But isn't that what the 'M' in 'LAMP' stands for? MySQL? Now, if the 'M' would stand for memcached, things would be better. Maybe it's time for the 'LAMMP' stack ;-)
The performance bottleneck for virtually every webapp is the programmer. In other words, doing stupid shit. You can write a performant webapp using Perl and flat files if you wanted to.
The most important thing is to know how to use your stack. It doesn't matter what the stack actually is.
For 99% of all LAMP implementations, the performance of the stack is not an issue.