After upgrading (Slackware-14.2/32bit) to seamonkey-2.53.1-i686-2 it takes a very long time to open the Mail/News window: over 30 seconds. Once that is open, the Calendar tab opens quickly, but it takes about 10 seconds to open the Tasks tab. Before, with Seamonkey-2.49.5, all of these were quick, less than 2 seconds.

If I disable the Lightning extension in Add-ons Manager, the Mail/News windows opens in 1-2 seconds like it did before the upgrade.

I thought maybe this was caused by Lightning now being used as a zipped file (.xpi), instead of being unzipped as in 2.49.5. So I unzipped it in the distribution/extensions directory and forced a reinstall to my profile. But this didn't help.

I'm back on 2.49.5 for now. (Remember: Backup your profile before upgrading. Also read the release notes: before upgrading, you need to clear your master password because some encrypted files changed format.)

Any ideas?

I removed the Slackware seamonkey-2.53.1 package and installed the 2.53.1 build from seamonkey-project.org instead. This one works fine, with no long delay opening Mail/News or Tasks. So I'm going to keep the "official" build from seamonkey-project.org instead.

I don't know the underlying cause. I did notice in about:buildconfig that the Seamonkey project uses ccache and gcc (6.3.1) vs Slackware use of clang. But I do not know if that is significant.