The title explains it... I don't understand why "cached" web pages take just as long to come up as the pages themselves; why does this happen?
Sometimes it takes a while for Google to retrieve that "cached" page. Other times what happens is Google caches only the HTML portion of a site. When Google serves you that page your browser parses it and tries to load any linked resources there, such as images or Flash objects. If the original site is down or extremely slow, even though your browser has the HTML part of that page, it still slowdown/stall when it tries to download linked images.
When that happens, sometimes Google includes a link to a "text-only" version of that cached page.