I've had this problem and it continues on and on for ages. It is based upon several things.
1) comments, pings, and trackbacks. The more links in the post, the longer it takes to load.
2) code processing - the more html, graphics, links and things you have in there, the longer the time it tends to take to get into the database.
3) post size - a 14 word post zooms in there, but a 300000 word post takes a "little longer".
4) Server speed - sometimes the server just slows down, like during maintenance, backups, or just busy times at the old server water cooler. No clue, just sometimes it is consistently slowing down at the same times every day - which implies server stuff.
5) Internet times of the day aka Internet Rush Hour - I find that during the day, my loading times are pretty fast, but in the evenings, especially popular sit at home and stare at the computer evenings, things load slowly.
6) Access Speed - obviously, the faster the Internet connection, the faster the load - but sometimes that tends to slow down for whatever mysterious reasons that haunt your modem or IP.
And the biggest criminal of ALL?????
7) WordPress's Edit Preview. Yes, the information may get into the database rocket fast, but the loading of the screen still has to load all that preview stuff down in the Edit Preview below the posting area. I think personally that this is a useful feature, but I can't tell you the last time I ever looked down there. The styles are not present - so all you see are the words and the pictures all looking pitifully gross in their unlaid out layout. I want to preview what it will REALLY look like, but this feature doesn't do that...yet. There is a WYSIWYG plugin, somewhere, that will actually show you the actual site from including the styles right in the edit window, but it slows everything down if you are doing massive stuff. So if you have a lot of graphics or ads in the post, you have to wait through them all loading up on the screen if you hit SAVE AND CONTINUE EDITING.
I've been able to speed things up during the massive edits and postings as I've been moving my static site into WordPress by turning off all pingbacks and comments, and I wish there was a way to turn on and off the Edit Preview area of the Edit screen, but the slow loading is hit and miss. Frustrating.
I've found that if you are tired of waiting, you can click the Manage screen if you are done with the post, and the post will still have already loaded up into the database - most important! You can go on. But if you hit the SAVE twice while you are waiting for it to load, it can and does double post your post, so you will have to delete one. Oh, yeah!