For me, the several seconds to minutes of being non-responding seem to correspond to when Thunderbird is checking mail, or receiving mail. Specifically, it often ends at the same time as the pop-up notification of new mail.

As if the program were single-threaded and was hung waiting for IMAP response.

Yes, something has changed for me. It used to work very well. We thought that Chrome was causing a problem but shutting it down does not seem to help.

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 post to let us know results.

- If no, then cause is either - bug in Thunderbird, in your Thunderbird profile, your mail provider. Please post into topic the contents of Help | Troubleshooting | copy text to clipboard

- If yes (problem is gone), then (still in Windows safe mode) ... start Thunderbird normally

    -- If problem still gone, then cause is a program loaded during Windows startup.  Possibilities include: antivirus SW, virus/malware, background downloads such as program updates
    -- If problem came back, then cause is likely a Thunderbird addon - eliminate them by disabling each one at a time in Tools | addons | extensions and restarting

- If results are unclear ... possibilities include temporary conditions such as contention from other running programs, downloads related to windows update, ...

Please let us know your findings.