On the Internet, every bit counts. Delays are common, but every saving in code (markup, style sheets) or processing time reduces latency.
Optimal performance on end user systems is important in multimedia design. Tweaking to reduce the resources required to run the product will help ensure the system remains responsive under stress—an important aspect for interactivity.
The nature of the web exacerbates the issue as the client must receive data from the server in timely fashion. This aspect of optimisation overlaps with accessibility concerns for users with low bandwidth (common in rural and remote areas).