Sounds like Anti-Virus is scanning incoming mail.

Access Anti-Virus and stop it from scanning incoming and outgoing mail.

How big are you folders? Do you regulary compact Inbox and Junk to remove all 'marked as deleted' emails? This will keep file smaller. Info on compacting - what it does and why it is important: http://kb.mozillazine.org/Thunderbird_:_Tips_:_Compacting_Folders

I regularly compact folders but I have years and years of emails so folders and profile are quite large. For super big folders, I broke them up into years in separate folders.

I am only running Windows10 with no AV other than Windows Defender.

It sucks because TB is impossible to use anymore. Wait, wait, wait.

I would add that I have hundreds of folders under around 6-7 different email IDs

Imap mail accounts can have some delay as synchronisation occurs for each subscribed folder. For old emails, which you do not want to lose, but do not need in Thunderbird, you can unsubscribe from seeing the folder, knowing if you really need access it is stored on server.

I discovered that I rarely needed those old emails but still had them sitting in folders forcing me to scroll more and more in the Folder Pane. For any account, I created folders to store old emails, then exported copies of folder to an external harddrive as backup or just in case I need access to them. Basically, I took old mail out of Thunderbird to reduce the clutter. You can either exit Thunderbird and access your profile folders, then perform a simple copy paste of mbox files or can use a good addon extension called 'ImportExportTools'.

re: I am only running Windows10 with no AV other than Windows Defender. You have something interfering and my first instinct would be AV or Windows very own 'Always on protection'.

Even a Windows or adobe etc update could cause a slow down.

Windows 10: Is it possible that you are using 'Always-on protection'? Good info here:

• https://docs.microsoft.com/en-us/windows/security/threat-protection/windows-defender-antivirus/configure-advanced-scan-types-windows-defender-antivirus

quote: Email scanning limitations We recommend using always-on real-time protection to protect against email-based malware. scans emails as they arrive and as they are manipulated, just like normal files in the operating system. This provides the strongest form of protection and is the recommended setting for scanning emails. Info on Windows Defender AV always-on protection and monitoring:

• https://docs.microsoft.com/en-us/windows/security/threat-protection/windows-defender-antivirus/configure-real-time-protection-windows-defender-antivirus

The links give info that says amongst loads of other info:

• The AV engine makes note of any file changes (file writes, such as moves, copies, or modifications) and general program activity (programs that are opened or running and that cause other programs to run)
• Downloaded files and attachments are automatically scanned. This operates in addition to Windows Defender SmartScreen filter, which scans files before and during downloading

I would advise you try to set it up so that Thunderbird files and folders are not scanned. Obviously, if you open an attachment then that would be different because it would be opened by eg: 'Adobe' or 'MSWord', so would be scanned anyway.

Explaining mail account 'folders'. Assuming you use pop mail accounts and/or imap accounts that keep messages on your computer; the folders you see in the Thunderbird Folder Pane are not 'folders' with emails as separate files; they are mbox text 'files'. For example: the Inbox mbox file is a single document with emails written to it, one after the other, in the order downloaded, so the oldest are at the top.

re :For super big folders, I broke them up into years in separate folders. What do you call 'super big' in MB? If you have a file that contains a whole years worth of emails, then it may well be still a very large single text file document.

Inbox: Always advise that the 'Inbox be used as an Inbox for new or recent emails that you need to read or deal with. It should not be used as a general storage. Please read good advise here:

• http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

It should be no problem having several mail accounts, but are they imap accounts that are performing synchronising on a lot of folders?

I'm offering you advise to reduce the clutter and stop Defender always on protection from constantly scanning.

dsooley said

I have gone through threads on lagging and cannot seem to get a solution. Basically, when a piece of mail is coming in on desktop, TB goes into FROZEN mode. Cannot click on or open mails, cannot type....it just becomes unusable and I need to sit and wait for up to a minute or more sometimes in order to use it.

This reduces the list potential causes - nothing to do with scrolling, etc. But still a sizeable list.

The two minute no muss no fuss test , without touching messages or folders, is

Start *Windows'* safe mode with networking enabled - win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode Still In Windows safe mode, start thunderbird in safe mode - https://support.mozilla.org/kb/safe-mode-thunderbird

Does problem go away?