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. Designers of Web technology will always have to consider efficiency. When designing formal languages to write resources in, designers should ensure that the languages are easy to parse and reasonably compact.
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. CSS likewise has several mechanisms for grouping rules. However, intuition isn't always a good guide when estimating efficiency. With SVG, the possibilities for that will be greatly enlarged.