I'm aware that with many tasks and calendars it can be a little slow, it's because it pulls all tasks from server every time. I'm trying to make it sync only changed tasks right now.
Improved sync speed in 45.1.8. But the main problem that all calendars fetched every time won't be fixed until I rewrite CalDAV implementation from scratch or python-caldav library will make that possible.
Is it just me or does the upload of tasks take some time? About 5 to 8 sec