High Sierra & Mail acting up

Since the latest upgrade - HS 10.13.1 - my iMac (Retina 5K, 27-inch, Late 2015) processor fan has started to act up on itself. Just opening a browser (Firefox) seems to be an effort for the iMac. Also - this is particularly annoying - Mail seems to be losing messages with attachments (pdf & excel, for example) on my Inbox. The messages show only Subject, no content, text or attachments.