Well, first, the upgrade to 1.5.1.2 helped tremendously. Much faster loading times.
Still, whenever I have a lot of links to graphics or external sites in a post, it can take a long time.
I found that the database gets the information IMMEDIATELY, but that to redisplay the page, WordPress has to go in and get the information and populate the form with it. The more information it has to put in there (custom fields, excerpts, lots of categories, and lots of pictures to load into the preview screen), the longer the page takes to load. Serious pain in the buns if you have ads like Amazon with in your posts, which we do on occassion for products we review.
I keep two windows with Admin information up now, plus at least one for my site. Yeah the power of Firefox Tabs! If it takes a while for a post to load and do its thing, I go to the other tab and check comments, and do other tasks, mult-tasking. There is no easy way around this.
It is a combination of server, internet, bandwidth, WordPress, pings and trackbacks, image links (internal and external), and relying upon the bandwidth, servers, and internet for those external links, especially with images.
I found in general that any external image link, like to an icon or logo, used within my site, the loading times of the site itself as well as the Write Post preview screen area (gosh, I wish we could turn that preview on and off!!!) takes FOREVER!
So the post information may save and reload but the preview helps slow things down, too. I wish there was an easy answer but it is a combination answer.