Categorizing Site Slowdowns

In my experience, there are three different categories of site slowdown and delay:

• Imperceptible
• Perceptible
• Unusable

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

Perceptible speed issues tend to turn into ones that make the site unusable (especially as larger numbers of users jump on the site) and Imperceptible delays tend to turn into perceptible ones gradually (as software is upgraded, as average server load creeps up, etc). It's often hard to tell what's causing an imperceptible delay.

Imperceptible delays have to be tackled proactively. You should check your site at regular intervals to see where things are. Upgrading through major versions of software (at any level in your software stack) should be considered carefully and after major upgrades (and after serveral cycles of minor upgrades) components of the system should be examined to find possible sources of delay.

Perceptible delays should be addressed immediately. If the delay is noticeable in the morning, then the delay will get worse throughout the day as more users hit the site. It's fortunate that the source of perceptible delays typically isn't hard to find, and the corresponding solutions are hopefully easy to implement (though not necessarily cheap...) something as simple as splitting the database onto a separate server, moving the site to a machine with less load, or upgrading hardware might be an easy solution to a general perceptible delay.

Delays which make the site unusable are -- by their nature -- difficult to predict sometimes, and in terms of importance have the priority of an outage. Other than 'fix them when they occur, and hope that you can prevent unnecessary ones', there's not much more to be said. A greater importance must go towards proactive maintenance and checking, especially if your application services many, many customers. This is one important area where web application development is not superior to developing traditional applications: the single point of failure.