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.