S'all about the contention ratio....
how many other people (just like you) are using the same service at the same time

ratios are
1:xx
Ie. under full load, every server in the cluster will be serving xx people simultaniously

"normal" isp's will offer webspace, incoming & outgoing mail servers with a contention ratio of about 1:50

Corporate packages are typically 1:20

Others include
1:10
1:5
1:1 (basically you're the only one using the box)

so,
lower contention ratio's
particuarlly for good performance at peak US usage times