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? Please reply to let us know results.

Thank you Wayne. A big improvement in download speed.

Good. In that case, try Windows and Thunderbird safe mode's seperately to see which one actually provides the benifit.

If it's Thunderbird safe mode then mostly likely the problem is a Thunderbird addon

If it's Windows safe mode then the most likely problem is AV or firewall software. (BTW, which AV software are you running??)

Please reply to let us know results.

I am having the problem that Thunderbird Ver 45.4.0 started running EXTREMELY slow when downloading messages after Windows 10 did an "update" that took over 5 hours. I don't know if this a Win 10 or a Thunderbird problem but expect Win 10 since I don't thing anything changed with T'bird recently. I am running Avast! Internet security, Ver. 11.2.2262 and their AV version 161023-0. This is a Samsung 64 bit computer.

n5jjr said

I am having the problem that Thunderbird Ver 45.4.0 started running EXTREMELY slow when downloading messages after Windows 10 did an "update" that took over 5 hours. I don't know if this a Win 10 or a Thunderbird problem but expect Win 10 since I don't thing anything changed with T'bird recently. I am running Avast! Internet security, Ver. 11.2.2262 and their AV version 161023-0. This is a Samsung 64 bit computer.

https://support.mozilla.org/en-US/questions/1141111#answer-922638

Thunderbird works well in the safe mode. I did not try starting Windows in the safe mode. Do I need to try that too? All Thunderbird add-ons are set to never activate. Restarted Thunderbird not in safe mode. It runs better but still not quite right. If not, how should I proceed now?

Looks like disabling Cloudmark DesktopOne fixes the problem. (discovered by first starting Windows in safe mode)