You need to click the box when you log in that says "remember me" to avoid this problem.
Taking a long time to submit a post and getting logged out isn't exactly a reflection of the forums inability to keep up with you, it is the fact that the same page stayed loaded on your computer and your computer either began to go to sleep or the amount of time exceeded your visit per your browsing allotment settings.
If you click the box to stay logged in you will have to manually log out or erase all of your history to get logged out.
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