spetterson said

I've just moved files from one PC to another. Reinstalled Thunderbird on new machine. POP3 account I've used for years. Have had slowness issues in the past on the old machine, tracked some to antivirus, etc. Old machine was behaving properly recently. The machines are somewhat different hardware, but both are Windows 7. I keep a big Inbox (easier searches); about 7000 messages. On the new machine, marking a message Read or Unread, or deleting a message, freezes Thunderbird for about 25 seconds. Moving from one already-read message to another has no delay. Only when moving off of a message that just got marked as read, or clicking to mark a message read, causes the delay. I tried moving half my messages into another folder and compacted the inbox; the freeze goes to about 11-12 seconds (proportional to inbox size). But I don't want to leave it that way or keep inbox so small. I compared behavior in other folders (I do archive files from 2013 and before into different folders): archive folders have the same proportional delay. Bigger folders can cause a delay of 40-50 seconds. Small folders have almost no delay. These things made no difference: - Thunderbird safe mode, no difference. - Turned off antivirus; no difference. - Turned on/off indexing (Windows indexing of folder, Thunderbird smart indexing); no difference. - Rebuilt global database; no difference. - Enabled "Use Thunderbird offline": no difference. - Verified Inbox gets rewritten instantly when I click on a message to read it or click to mark unread. the freeze comes after that. This, however, eliminated the delay: - Rebooted Windows in safe mode with networking. Thunderbird responded instantly, as it behaved on the old computer. Delay was gone. Rebooting the computer back into normal mode, the delay returns.

It sounds as though your computer has just slowed down. Each day they slow more and more. You may need more RAM? With 7000 messages in one folder, they are not going to show all at once. I would think this is normal. RAM has become so inexpensive theses days, I always max out my PC's with it. You may of answered your own question when you say the bigger the folder, the slower it is.

When I reboot the computer in "safe mode with networking", Thunderbird behaves correctly *without delay*. On the old computer, with less RAM, slower drive, slower CPU, the folder behaved correctly *without delay*.

It is the new computer that has the delay. The old computer does not have this delay.

The problem is not that the folder is large. The problem is that after marking the message read or unread, Thunderbird (or, more likely, the OS, upon request from Thunderbird) is taking some other action that it does not need to take, and that it does not take in my other, intended to be identical, configuration.

re :When I reboot the computer in "safe mode with networking", Thunderbird behaves correctly *without delay*.

This tells you that another program outside of Thunderbird is causing the issue. Cause is likely a program loaded during windows startup. Possibilities include: antivirus SW, virus/malware, background downloads such as various program updates.

re: I keep a big Inbox (easier searches); about 7000 messages. On a different note.... The Inbox folder as seen in the Folder pane is actually a simple mbox text FILE . Emails are written one after the other in the order downloaded in the same file.

So all of those emails are actually stored in one file - a single document, not individual email documents in a folder. Whilst this is ok, it is not advisable and carries an element of risk.

• http://kb.mozillazine.org/Keep_it_working_%28Thunderbird%29
• http://kb.mozillazine.org/Performance_-_Thunderbird

I found my problem - and thank you to everyone who responded.

It's my own fault - I said it wasn't the antivirus, and that's because I had turned off all *internet* monitoring, and that didn't affect it.

Explaining it to a friend in person, I realized I hadn't tried turning off real-time *file* monitoring.

That was the problem. McAfee was scanning the inbox for file attachment viruses every time the file was rewritten (every time a message was marked read or unread).

Obvious once I thought of it. Thanks, everyone. Problem is resolved.