Time is money. No doubt about that. The more time your customer waits for your service response the less coins may hit your pocket. You may ask: why? If your page loading is too slow your "almost" customer hits back in his browser and selects another link from sponsored links Google provides him. You loose this customer in favour of another supplier.
What about Google? Was their service (no kidding, it is a sevrice that searches information, paid by ads) always so blazingly fast? Let's check this site performance monitored from London server over few years:
As we can see there were some small problems in 2009 (~5% of requests required more than 1 second to be processed). Not so bad.
In order to double check our measurement results there are monitor result from Newark, USA:
As you can see there were also (small) performance problems in 2009, in some months ~2.5% of requests were processed in more than 10 seconds. Still pretty damn good.
You can see current stats for google here, thanks to site-uptime.net - site uptime monitor.