I am using Thunderbird since a long time and never had real problems with it. This recently changed on one of my PCs (Windows 10 x64 Enterprise, Thunderbird 60.9.0 32 bit).

On this PC, Thunderbird ran smoothly for several weeks. That was in March / April this year. Then, all of a sudden, it started to lag after certain actions. For example, if I delete a message from the message list by selecting it with the mouse and then hitting del, it lags for up to five seconds before it actually moves that message to trash.

It never freezes, though, and other actions are carried out as usual. There is no lag when operating the menus, for example.

This happens with IMAP folders / accounts. However, I am absolutely sure that the IMAP server is not the problem because Thunderbird does not show that misbehavior on three other PCs which are using the same IMAP server.

So far, I have taken the following steps to solve the problem, but without success:

  • Deleted the .msf files in the Thunderbird profile;

  • Disabled the global search in Thunderbird options;

  • Disabled all addons / extensions (except Lightning, because I am not sure if it will keep all data when being disabled);

  • Disabled Windows indexing globally.

The only thing which I did not try is disabling the virus scanner. I am relying on Windows Defender, and I actually don't know if it interferes with the TB mail boxes at all and if I could exempt some folders from real-time monitoring. I am currently researching this, although I don't believe that the virus scanner is the problem; after all, other file operations (using the Windows File Explorer) are carried out instantly.

The PC in question is relatively new and powerful (Xeon E3-1230v5, 16 GB RAM, reasonable mainboard and SSD).

So what else could I do to eliminate those lags?


In the meantime, I have uninstalled Thunderbird completely, then installed the 64-bit version and created all accounts and other settings from scratch. To my surprise, the problem persists.

Now I am completely baffled.

EDIT 2 - (Sort of) solution

In the meantime, I have learned to disable Windows Defender or to exempt TB's profile folder from Windows Defender's real-time monitoring, respectively. After having done that, the lags did not occur any longer.

Although the cause of the problem is now known, I still don't have a satisfying solution. I have found some articles which deal with the same problem and which recommend exempting TB's profile folder from Windows Defender's real-time monitoring as the final solution, stating that this would not mean less security because a malicious email attachment would have to be saved elsewhere or be executed to become active or harmful.

I am considering those statements dangerous nonsense, because there are much more other malware sources than email attachments. If you exempt a folder from your virus scanner's real-time monitoring, you are inevitably providing a place where any malware (notably malware which does not come as email attachment) can install itself.

Given TB's market share, I am quite sure that malware authors soon will try to have their products installed to the TB profile folder, knowing that many people who run Windows Defender have the problem described above and thus will disable real-time monitoring for that folder.

Hence, I definitely will not implement that sort of "solution". The only safe solution obviously would be switching to another antivirus software which can protect TB's profile folder without introducing that annoying lags.

However, this isn't satisfying either because it means quite a big change to a running system. I'll have to think some days over the situation and eventually will stick with the lags for the time being.

  • Do you have something like that in web browser? [It would suggests DNS related problems]
    – AnFi
    Sep 13, 2019 at 9:56
  • No. I am using Firefox and have absolutely no problem with it.
    – Binarus
    Sep 13, 2019 at 10:14

3 Answers 3


I am relying on Windows Defender, and I actually don't know if it interferes with the TB mail boxes at all and if I could exempt some folders from real-time monitoring. I am currently researching this,

And your results with Windows started in safe mode (step 3 of https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems ) was ... ?

I don't believe that the virus scanner is the problem; after all, other file operations (using the Windows File Explorer) are carried out instantly.

Unfortunately that is not a sufficient measure of the potential impact of Defender to Thunderbird. See https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues Also, a recent example of a report is Bug 1559677 - Intermittent Lag composing new email. Low/no CPU. But MsMpEng.exe (Windows Defender) uses a full core CPU

  • 1
    Indeed, Windows Defender seems to be the culprit. In the meantime, I have learned how to disable its real-time protection and how to exempt the Thunderbird profile folder from real-time monitoring, respectively. In both cases, the lags were eliminated. So I am giving the answer and +1 to you.
    – Binarus
    Sep 16, 2019 at 13:03
  • However, knowing the reason in this case does not mean having a solution. Many articles recommend exempting TB's profile folder from Windows Defender's real-time monitoring, stating that this wouldn't mean less security because malicious attachments can't get harmful before they are saved elsewhere or get executed. I am considering this argument a dangerous nonsense because -independent of email attachments- any malware could install itself in the not-any-longer-monitored TB profile folder then. Only safe solution: Other virus scanner (not being keen on that ...).
    – Binarus
    Sep 16, 2019 at 13:11
  • 1
    Then you don't understand how Viruses and AV work. 1. Thunderbird doesn't execute CODE out of it's data profile directory. 2. If a virus is *contained in Thunderbird data files in the profile directory, and something (thunderbird or some other program) manages to launches it then it is caught by AV's realtime checker, 3. f a virus is installs itself ed in a Thunderb
    – wsmwk
    Sep 18, 2019 at 3:56
  • Then you don't understand how Viruses and AV work. I think I do. I have explicitly talked about malware which is not getting onto the system via Thunderbird. For example, a drive-by-virus getting onto the system via the browser and installing itself into TB's profile directory, or malware which at first is saved on the system and isn't recognized by the virus scanner due to some exotic packing, then unpacks itself directly into TB's profile directory.
    – Binarus
    Sep 20, 2019 at 15:03
  • Excluding something from Scanning, which is what is recommended for Thunderbird directories, does NOT prevent it from being monitored by AV as an executable (yes, not) - regardless of where it is installed.
    – wsmwk
    Sep 22, 2019 at 0:36

I am not sure if I should have posted this as an update to my question, but here we go:

  • In my case, it is definitely Windows Defender which causes the lags (already stated in the comments below the accepted answer).

  • I also have learned something new:

In the meantime, out of curiosity, I have uninstalled and re-installed TB a few times, each time deleting the local IMAP mail folders and files (*.msf and friends) after uninstalling.

Each time after reinstalling, I let it re-download (synchronize) all IMAP messages, and each time, there were no lags at this point. Then, each time, after a while of usage or after closing and opening TB, it offered to compact some folders.

If I accepted that offer, this was exactly the point where the lags returned. In other words, there were no lags in a freshly installed TB, but as soon as it had compacted folders, the lags were there again.

As stated above, I have uninstalled and reinstalled a few times and observed that behavior every time. It may be pure random, but I personally believe that compacting at least makes the situation much worse, so I thought this might be worth mentioning.

Note: Windows Defender was fully active during these tests; notably, I had not excluded TB's profile directory or parts of it from real-time monitoring.


@binarus I encountered the same problem when I recently updated from Win7 to Win10 and to Thunderbird 68. Thunderbird was also slow to start.

I can add the following and consider the problem solved.

• For me the lag occurred only when deleting from my Inbox.

• I watched my Task Manager during deletes and noticed the Antimalware Service Executable / Windows Defender Antivirus Service jump to the top for the few seconds that the delete took.

• This confirmed that as a source of the problem so I followed your suggestion, except that I had Windows Defender exclude only my Inbox file in my Thunderbird profile. The delay went away.

•• Here's the clincher: when I removed the exclusion the delay stayed away.

Oh, and at some point in between I restarted Thunderbird. I don't know if that's relevant or not but the restart was back to normal too.

  • 1
    Thanks for the answer. However, I am pretty sure that the delays will return some day in your case, at least if you decide to compact your inbox. I am still suffering from them and have decided to live with that situation for the time being. The size of the inbox (number of messages) probably is also important.
    – Binarus
    Jan 22, 2020 at 7:59
  • Well, it's been nearly 10 days now, with lots of comapactings, restarts and emails in and out and I've had no recurrence. All I can say is thanks again for posting about this. I owe you one. It would have driven me nuts.
    – VanderM
    Jan 27, 2020 at 12:41

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