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Reason being that there is a lot of javascript processing happening on the page. You click a "edit this topic" link and you lose all of that spell checking, link checking, duplicate topic checking, etc. When you realize that you opened the topic in the same tab to edit it, and click back, the page has to do all that client-side work again, and it takes a significant amount of time.
*** This bug has been marked as a duplicate of bug 970807 ***