The Log-Normal distribution is the one I find best at describing latencies of server response times across all the user base over a period of time.
A log-normal (or lognormal) distribution is a continuous probability distribution of a random variable whose logarithm is normally distributed. Thus, if the random variable X is log-normally distributed, then: $$Y = ln(X)$$ has a normal distribution.
You may see some examples at the aptly-named site lognormal.com whose in the business of measuring site latency distribution over time and more. I have no affiliation with the site except for being a happy user. Here's how the distribution looks like; response (e.g web page load) time vs number of responses:
(1) In this chart, the load-time (X-axis) scale is linear. If you switch the x-axis to a log-scale, the shape of the distribution would look more "normal" on the right side of the peak.
(2) A log normal distribution value is always positive. Obviously, a server can't respond in negative time.
(3) The log normal distribution is right-skewed. The longer tail is on the right of the peak.
(4) Update 2023-08: lognormal.com has been acquired by Soasta, which was later acquired by Akamai, so the original lognormal.com page no longer functions as it used to when the original answer was posted. Instead it redirects to some Akamai page. The example server-response-time distribution (CDF) chart was saved from lognormal.com years ago when the service was still active and remains useful (it is hosted on i.stack.imgur.com)