See https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems#Diagnosis_Steps

Thanks for the link...I tried most of the things on the list and the performance is still really bad. For an application that doesn't do much, it sure has become a performance hog since version 38.x.x. I'm at the point of either reverting back to an old version or switching to a different email client.

I did find that the Inbox file (I moved the default directory) never got smaller. It stayed at 1.5GB. I had to move the emails to another folder and delete the Inbox for the iMap account.

I didn't see anything about reseting all settings to the default, is there an easy way to do that before I revert versions?

YOu know mail files get smaller when you compoct, that is it's purpose and most people that complain about files not shrinking also reject the requests Thunderbird makes to compact. Are you one of them.

As for 45 being worse that 38. I seriously doubt there is a technical reason in Thunderbird.

Now you have moved the folder to somewhere else. Is windows search is indexing away at it? That is a major cause of the application almost grinding to a halt.

Have you created a exception in your anti virus for the profile folder? Again having an anti virus spending 10 minutes scanning that 1.5 Gb file every 10 minutes when you get a new mail is also something that can slow things to a crawl.

Have you tried the two safe modes mentioned? Thunderbird add-ons can have a very deleterious effect on performance, especially if you have one that was updated and now has a but. We saw this in the past with lightning. It worked fine unless you had the birthdays calendar enabled from Google.

I am asking because you don't say what you have done, just most of them. I would also guess most does not include the things external to Thunderbird because people tend to focus on what they think is the problem.

Thanks for the suggestions, Matt.

I compacted all folders manually. I may have disabled the automatic compacting by mistake.

Would you recommend a clean install of thunderbird? Or, Is there a way to revert all settings back to the default without loosing my server/folder settings?

I'll try the Symantec SONAR exception for the folders and see if that is the cause. When I have the performance issue, the thunderbird application is the app that maxes out the core in the task manager...would symantec SONAR run inside of the thunderbird thread?

I'm just wondering why this wasn't an issue with my earlier thunderbird version. I never had a performance issue like this before.

I'm running thunderbird in a VirtualBox Windows 7 SP1 64-bit, with 1.25GB Ram for the client host. The host is a Mac book Pro.

Here is my list of what I've tried based on the link you originally sent. 1) Yes, 2) tried, but still bad performance 3) Running Windows with Symantec -I just added an exception for Symantec SONAR 4) N/A 5) N/A 6) all searches disabled, including moved mail folder 7) index disabled in mail folders 8) memory is fine 9) tried recreating global-message-db.sqlite 10) see 9 11) I manually cleaned the iMAP inbox which was 1.5 GB 12) disabled interval checks on imap account 13) ?? 14) N/A 15) no, it's small 16) N/A 17) No, updater.exe is not running 18) N/A 19) it's on a SSD, defrag is N/A 20) N/A 21) N/A 22) N/A