Thunderbird slow opening emails in large folders after recent BitDefender update

After a recent BitDefender update that required restarting the PC, Thunderbird has got slow opening e-mails in any large local folder, such as with a few thousand e-mails or over 1GB in size. For example, my Inbox (a basic local folder, not IMAP or synced) is 3.7GB and any unread e-mail I open shows "Loading Message..." for about 8 seconds, regardless of whether the e-mail has attachments. During this waiting time Task Manager shows Bitdefender Virus Shield using a steady 14.5% CPU. Marking e-mails as Unread/Read in large folders has the same effect. When I disable Bitdefender Shield, e-mails open instantly like before this recent BitDefender update.

I tried reinstalling BitDefender and the problem went away temporarily. About an hour later a pop-up said BitDefender needs to restart my PC to finish an update. After I rebooted, this issue returned.

I'm using the BitDefender Free Edition (Build 26.1000.32.77), Thunderbird 102.9.1 and Windows 10 Pro x64 22H2. PC is a HP Z240 with an Intel 7700K i7 CPU, 1TB Samsung 970 m.2 EVO SSD and 32GB of RAM.


  • Yes, created an account here to report the same behavior. Same BitDefender version, Thunderbird version, and Windows version.

    I tried several resolutions with Thunderbird itself to no avail - very slow, even (Not Responding) freezes in Thunderbird just navigating emails.

    Uninstalling and re-installing Bitdefender temporarily downgraded the version below 26.1000.32.77, at which point Thunderbird was no longer delayed or frozen.

    Unfortunately Bitdefender wants to automatically update so while writing this it has updated to that build again. Navigating email is very fast right now - as soon as I restart to take the update, I expect that to take a hit.

  • genpix
    edited April 2023

    confirm the identical behavior (same Windows, Thunderbird and Bitdefender versions, Samsung SSD for C: drive and Thunderbird Profile).

    Though, want to add some details on behavior:

    When I delete an e-mail message from inbox (Thunderbird actually moves this e-mail from Inbox to Trash folder), Thunderbird freezes for several seconds (5 or so seconds). Deleting the same e-mail from Trash folder (actual deleting from local profile folder) does NOT cause the freezing. It looks like the moving of unwanted/spam e-mail into Trash triggers the whole chain of events in Bitdefender, and as a result 5+ seconds freezing in Thunderbird.

    Temporal disabling of Bitdefender Shield eliminates all Thunderbird freezing. In contrast, adding Thinderbird Profile location (entire profile) into exception list does not help with freezing at all. I am only able to select Antivirus and Online Threat Prevention for thunderbird profile location in Exception options, though. Advance Threat Defense option is greyed out (may be this is a reason why adding this exceptions does not have affect on freezing).

  • I decided I'll try cleaning out some old e-mail from my Inbox and didn't realise how long it was going to take to clear out a bulk selection of 500 messages. About 10 seconds later it showed "Moving 2 of 500 messages...", so I started a stopwatch. It took about 25 minutes to move these messages, mostly Amazon newsletters and not a single attachment in any.

    I then disabled Bitdefender shield and timed Thunderbird clearing out another 500 messages. It finished in just 15 seconds.

    I also tried turning off various Defender components such as Email protection, Exploit Detection, Web attack prevention, etc. temporarily, but they had no effect. Only turning off Bitdefender shield restores Thunderbird's performance.

  • Same problem, since the recent BD update. Incoming mail seems to arrive normally, but when clicking a message to read, Thunderbird goes non responsive for 30-90 seconds, on every new email. Once new messages have been read, I can usually (not always) swap between read messages with no hangups. Sending emails also causes TB to hang. I completely uninstalled BD and reinstalled, no change. Disabling BD for any amount of time, Thunderbird works perfectly.

  • small correction: freezing (during e-mail deleting) is more like 8 seconds, not 5. This is VERY annoying.

  • Alexandru_BD
    Alexandru_BD admin
    edited April 2023

    Hello everyone and thank you for taking the time to share your observations here.

    The common denominator here seems to be Antivirus Free and the affected version 26.1000.32.77, as I haven't seen any similar reports for the other solutions. I have reached out to our development teams and requested an investigation. Please standby for updates, as I will post my findings here once the tests are concluded.

    Thank you for your patience and understanding.


    Premium Security & Bitdefender Endpoint Security Tools user

  • genpix
    edited April 2023

    I rebooted my PC today (April 10) to take new Windows update (KB5023696), and ... looks like Thunderbird "freezings" are gone. I am now trying to collect more info. Is it really KB5023696 Windows update? is it windows reboot alone? is it something else which stopped BD-induced TB-freezing?

    PS: a week ago, I tried to uninstall the most recent Windows update (I am not sure what number it was, it 's possible that it was the very same as today's one) with a hope that it was a Windows update who cause the havoc with TB. Did not help at all (thus, I assumed that it was not a cool-print). Today, I "re-installed" it (or may be Microsoft added some addon in the mix?) and TB looks much happier now (BD is still on the same build, 26.1000.32.77)

    PPS: one more thing, my Windows 10 is still on version 21H2 (not the most recent 22H2). Particularly, Windows 10 Pro, version 21H2, build 19044.2728

    Question to those who observed the freezing, have you tried simple re-reboot again (after installing BD update during previous reboot)? Maybe, double reboot is fixing the problem? Unfortunately, my second reboot was together with Windows update, thus I can not answer my own question (by the way, I am one of those geeks who never switches off a PC. My Linux boxes are running 24/7 without reboot for many months. Microsoft is annoying in this regard: they want to reboot Windows too often, unless you disable updates).

  • editorsean
    edited April 2023

    I tried manually checking for updates and it installed KB890830 (Malicious Software Removal Tool v5.112), KB5025915 (2023-04 .NET 6.0.16 Security update) and KB5025221 (2023-04 Cumulative Update for Windows 10 Version 22H2). After the update did a Windows restart, Thunderbird continued to freeze for around 5 to 6 seconds opening each unread e-mail. I also tried rebooting two more times before opening the last few newsletter e-mails, which also took about 5 seconds each. So unfortunately no improvement here with my PC.

    Here I'm running Windows 10 Pro x64 22H2. Going by my Windows update history, it installed KB5023696 on the 16th March.

  • Restarted Windows 7 after the latest update on 4/10. Thunderbird still hangs when opening messages.

  • genpix
    edited April 2023

    forgot to mention:

    Before reboot, I added entire TB profile folder to the exception list (Online Threat Prevention + Antivirus On-access scan). These exceptions had no effect at all (before reboot). Bitdefender Shield On/Off was the ONLY switch in BD which IMMEDIATELY affected freezing. After reboot (described above), I still have these exceptions settings for TB profile, and BD Shield is still On.

    I wonder, may be exception settings in TB don't take immediate affect? Is Reboot (or BD restart) required for exception settings change?

  • I've tried implementing your procedure but it does not help in my case. They only way Thunderbird acts normally is when Bitdefender AV is completely turned off.

  • genpix
    edited April 2023

    @montalivet, you may need to reboot after you made the changes described above (reboot was required in my case; though, it does not mean we had the identical initial configuration).

  • spoke too soon :(

    after the last Windows update (and reboot), TB freezes again on both, access and delete the e-mails from inbox(es). Exception in BitDefender don't help anymore. Once again, disabling Bitdefender Shield is the ONLY way to stop TB freezing.

    I am at the point of uninstalling BD :(

  • Good day

    I'm getting the same issue, do you have any update for this?

  • genpix
    edited May 2023

    looks like new version of BD (build is fixing the freezes.

    I don't even need to put TB folder in the exception list anymore.

  • With Build 26.1000.32.85(up to date) same issu.

    Added TB folder to exeptions solved....