Thunderbird Slows Down To A Crawl When Downloading Msgs

[MozillaUser9000]

Hello,

I have Thunderbird on two setups on my pc, two separately powered, bootable hard disks. Like two pcs, but all same hardware except hard disk.
Both are Windows 7 Pro 64 and both have the same problem.
When I start to download emails, Thunderbird grabs many per second. But when I get to about 500 emails, it begins to slow down and the more it downloads, the slower it becomes.
After downloading about 3000 emails is takes 40 seconds for one email to download.
This happens with two different email providers.
Both are IMAP interface.

I have already tried disabling add-ons.
I have tried Silent Mode with my Norton Internet Security.
I have tried the suggestions here... https://www.faqforge.com/windows/how-to ... windows-7/
I have tried reinstalling.
Problem just persists.

Anyone have any ideas ???

[wsmwk]

Started Windows in safe mode?

[MozillaUser9000]

Just tried it.
No change. Still bogs down to a crawl.