When I fix sites, visits go up 1,000% and sales go up 500%. The typical web server has about 40% capacity. That is how you lose those $100/head visitors plus a lot of existing customers.

If you have one web server and can measure any activity, buy 3 more servers.

Forget CPU usage, the heaviest scripts use about double what you use now. The heaviest database usage is around 6 times what you use now. IO is more of a problem than CPU.

If you use SQL Server, it is badly tuned. I can say that with confidence because there are just not enough people with the right experience to tune all the SQL servers out there.

If you use OOP or any sort of middleware, you have a performance problem. I refer to a paper I presented to CMG way back when IBM introduced a product named DB2 and suggested there might be a way to distribute processing.