In my experience, there are three different categories of site slowdown and delay:
Other distinctions really don't matter so much. The user doesn't care whether the page load latency is 10 seconds or 15 seconds. They're just going to leave.
The progression of a delay is:
Imperceptible -> Perceptible -> Unusable
There are a few simple steps you can take to optimize Drupal. Drupal is a large, heavy system: every advantage counts.
This initial benchmark is taken without any of the variables which I'm going to test: css aggregation, mod_deflate, and apc.
ab -n100 http://drupal.shadowlife.ca/drupal-5.7/
Connection Times (ms)
min mean[+/-sd] median max
Connect: 0 0 0.0 0 0
Processing: 24 40 20.4 31 123
Waiting: 21 34 17.2 28 119
Total: 24 40 20.4 31 123