I think it's when you have processing rules in place for some or all folders, it seems they get applied to all messages again every time you open a folder. Norm

I think it's when you have processing rules in place for some or all folders, it seems they get applied to all messages again every time you open a folder. Normal filter rules don't expose that behavior, and I've been too lazy to look up what the difference is supposed to be. However, ever since I only add rules to "configuration" -> "filter..." I can switch between 5k+ message folders instantly (using IMAP).