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Slow downs - may be caused by potentially performance configurations by the SharePoint admin (for ways to get the server changes made ref: Fine Tuning Your SharePoint 2010 Environment – Front End Optimizations) or by having lists with membership changes where the list no longer has (or percieves to have) valid users referenced by the list items (people or group pickers) or if you have a bad list ( or corrupt one) and the SharePoint reference is getting lost becuase it can't find home. In my experience I have found a few things cause the slow down and login behaviors raised by the original question.
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