Opening up Thunderbird has become exceedingly slow. It takes 15 minutes to download my gmail. And during the day it often says it cannot connect to gmail, then says that the connection is reset. What has gone wrong?

What is your antivirus software?

McAfee

McAfee has been problematic in connection with Thunderbird in the past. Try to start [https://support.microsoft.com/windows/start-your-pc-in-safe-mode-in-windows-92c27cff-db89-8644-1ce4-b3e5e56fe234 Windows 11 in safe mode] with networking enabled. Does the problem go away? Windows safe mode disables anti-virus software. This is a troubleshooting step, not a solution.