To debug startup performance, I would see if having one account instead of five makes a difference. I haven't used Thunderbird for a long time but if the delay

To debug startup performance, I would see if having one account instead of five makes a difference. I haven't used Thunderbird for a long time but if the delay is due to slow responses from one mail provider in particular, perhaps you can set that account to be offline by default and explicitly take it online and check for mail after Thunderbird starts, while you check your other accounts.