This might be caused by a Mail bug where it’s unable to handle AppleScript commands while using a Mail extension. You could try unchecking the SpamSieve extension in Mail’s settings:
but leaving Enable spam filtering using Mail extension checked in SpamSieve’s settings:
(SpamSieve will still be able to filter your mail using other means.) If that helps, great. If not, please record samples of Mail and SpamSieve during the hang so that we can see what’s causing it.