What is your antivirus? If you have an app scanning mail, it often becomes incompatible with a TB update, causing slowdowns. Note that to change from POP to IMAP, it's not sufficient to just change the server settings - you have to add the account as an IMAP account.

Running windows antivirus... Yes when I changed from pop to imap I did create a new account. But now you gave me three possible sources of the problem... TB, Comcast, and Windows security. I have not changed settings for several months... and not since it was transferring fine. What I can not do is find a way to isolate the issue... Oh it has become slow on all of my three boxes (all at the same level for OS and TB).

You could run Windows in safe mode to see if any startup apps are interfering with TB. This is the usual way to see if security/AV apps are the source of issues. In the case of Windows Defender, I'd recommend adding the TB profile folder as an Exclusion in WD (Virus and threat protection settings).

If your Comcast settings are correct and worked before, there could be temporary issues with Comcast servers.

As for TB, run it in safe mode to test if any part of the update, e.g. add-ons or layout, have affected performance. Hold Shift when you launch TB.