What is your anti-virus software?

I'm using McAfee LiveSafe

Try to start Windows in 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

Does the problem go away?

Yes it did...

So your problem is almost certainly caused by McAfee. McAfee is know to cause problems. See https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#McAfee

Or get rid of McAfee altogether.

The list of AV software that doesn't cause issues is rather small for Thunderbird. I have been on with McAfee and they checked the settings for their product, removed and reinstalled the product but to no avail.

I will have to find another email client, but thank you for your help!

You shouldn’t really need to find another client, and basically do not need any anti-virus product interfering with any email client at all.

The Rebuilding the Global Database support article suggests to just remove the global-messages-db.sqlite file in your Thunderbird profile folder when slowness occurs. Did you try that?

Additionally, general advices with regard to Thunderbird’s performance are

1. Disable Global Search and Indexing in Thunderbird’s Options > Preferences > Advanced panel > General tab > Enable Global Search and Indexer
2. Disable email traffic scanning in your anti-virus software
3. Disable Windows 10 indexing (how to)

See also: http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Anti-virus_Software#Antivirus_program_settings http://kb.mozillazine.org/Email_scanning_-_pros_and_cons

Echoing Tonnes, you shouldn’t really need to find another client.

If McAfee is impacting Thunderbird, then a) disable Windows indexing and b) set McAfee to EXCLUDE everything in the https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data