Web app scale bottlenecks (software & hardware) have leapfrogged themselves a few times over the past 15 years, and I just came across a concise view of how software had to react once hardware/bandwidth reached a new speed/throughput tier back around the year 2000; http://www.kegel.com/c10k.html
The C10k problem, and solutions, document is a fabulous overview of the techniques used to eliminate software servers as a bottleneck. Thus, hardware became the bottleneck again.
If you can't meet your application's user demand it is because you haven't written the correct software to do so. You can try to blame the fact that you don't have enough hardware/bandwidth, but that problem is solved with the flick of a pen (money). Your challenge is solving your software problem. Enough components in the stack (from web servers, to application servers (custom if you have to) and data storage solutions) solve the C10k problem now, that any issues in your application software are likely yours. Find better tools/components, and/or write better code.
If you don't have the money to acquire the hardware you want, you might be working on the wrong thing.
