About a week ago my Lightning app was updated to 5.4.0.1 ; immediately Thunderbird slowed to a crawl. It creeps while connecting to the Outlook server; after connecting, it takes a painfully long time to open the Inbox. Every step is very slow, and very frequently stops, showing a message stating: "Connection To Outlook Server Timed Out." My OS is Windows 10 Home version 1607; my computer is HP Pavilion dm4 with Intel Core i5, 2.50 GHz, 64 bit operating system, x64 bit processor, etc. I have read a number of similar complaints, questions and answers on this site. None of those comments have offered a procedure that works, other than those that suggest disabling Lightning. This works! But then, I have no Lightning Calendar! I have also verified my servers imap and smtp connection parameters, etc. Is there a solution for regaining the speed of Thunderbird, while enabling Lightning Calendar?