Hell yes, especially during Calendar syncs. Also: startup time. I completely agree. I have so many calendars that after launching Thunderbird, it takes around

Hell yes, especially during Calendar syncs. Also: startup time. I completely agree. I have so many calendars that after launching Thunderbird, it takes around 3 minutes 30 seconds to become responsive to user input, pegging a CPU core the entire time. And when calendars sync while the app is running (which happens all at once rather than being staggered,) it becomes unresponsive for about a solid minute, again pegging a CPU.