Comment 70 for bug 1458593

One potential workaround:
I am using the same set of CalDAV calendars (with lots of events, tasks and alarms both single and recurrent ones) on a number of machines each with Thunderbird/Lightning (I love it). Surprisingly on my slower computers the sync and Thunderbird/Lightning were fast and the GUI smooth but on my fastest machine each start of Thunderbird and each (also periodic) sync of the calendars took forever (approx. 10min...). In addition the GUI blocked each time I made a change to something within any calendar. What resolved the slowness problem on the fast machine was simply (i) to unsubscribe from all my (CalDAV) calendars, (ii) to restart Thunderbird and (iii) adding the same calendars again by subscribing them in the same way as before. Clearly if your problems are due to intrinsically inefficient algorithms in Lightning and your machine is just to slow to process those in time then this workaround does not help you. By the way: Emptying the cache of the calendars (they are configured to have offline support) did not resolve my issue; only unsubscribing, restarting TB and subscribing again did.