Performance improvements for Drupal
Part of the history of Drupal is the claim that it has survived slashdottings without knuckling under. Slashdot is a popular site among geeks and often when something reaches the front page hordes of geeks follow that link all at once. This has been known to bring weak web servers to their knees. Regardless of whether ones sites are slashdot fodder, it's a good idea to use the least machine resources to serve up your web site. Not only is it good for your wallet (fewer servers to buy) but it's good for the planet (fewer servers chewing up electricity).
Of importance is my earlier research, Website usability and performance
If your website appears to be "slow" your visitors will tend to be frustrated and go elsewhere. Performance can have an impact on your bottom line.