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are 2 possibilities:

1. Your forum it's to big or not optimized (table optimized)
2. Timeouts are to small in your php.ini file

Tell me if the xml file it's generated very fast or you wait a time until is generated.
My sql querys are quite equal with your sitemaps sql querys ... but are several querys in plus who help for a good aprox for "change freq" and "priority" in google sitemap ...