According to Jakob Nielsen, people are most productive if the computer's response to their click takes less than a second. People lose their concentration if a page takes longer than ten seconds to appear.
Of course, computers get more powerful and bandwidth increases continually, at least on average. But at the same time new devices are being connected to the Web, such as mobile phones and TVs, and they are much slower. They too will improve, eventually, although they will probably always be behind desktop computers. But then there will no doubt be new appliances (and applications). Designers of Web technology will always have to consider efficiency.
Speed loss can occur at the server, if producing a response to a query is too complex; at the network, if there is too much data; and at the client, if visualizing the data is too hard.
When designing formal languages to write resources in, designers should ensure that the languages are easy to parse and reasonably compact. "Progressive rendering" (the effect that displaying a response starts before all the data has arrived) can help as well. CSS, for example, has been designed in such a way that every line of a document can be displayed as soon as it arrives (although for text inside tables that requires some extra work by the style sheet writer). Caches help as well, and thus splitting documents in cachable and uncachable parts is a good thing. E.g., HTML and XLink allow documents to be assembled from parts, some of which may be shared among several documents. Sometimes facilities for compression or switching to a binary format may even be needed.
It is in general a good idea to provide methods in a language to remove redundancy, (although they have to be weighed against the added complexity of the language). Thus HTML and SVG have a mechanism to assign a "class" to an element and define the style for all elements of that class in a single place. SVG also allows re-use of graphical objects in multiple places: drawing a 12-point star thus only requires a single point, displayed 12 times in different places. CSS likewise has several mechanisms for grouping rules.
However, intuition isn't always a good guide when estimating efficiency. Network throughput isn't linear: a message consisting of a single TCP package is much faster than one with two packets, but four packets typically take less time than two separate messages of two packets (see [Frystyk97]). Java is often perceived to be slow, but Jigsaw (an HTTP server in Java) can typically hold its own against Apache (an HTTP server in C), because much of the perceived slowness comes from starting the Java virtual machine; once that runs, Java is fast enough (see [JigPerf]). And as a W3C study showed some time ago, most gain comes from radical changes: replacing GIF with PNG and HTTP/1.0 with HTTP/1.1 certainly helps a lot, but not nearly as much as replacing an image by some text with a style sheet. With SVG, the possibilities for that will be greatly enlarged.