1. Start Windows in safe mode with networking enabled - https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode 2. Still In Windows safe mode, start Thunderbird in safe mode - https://support.mozilla.org/kb/safe-mode-thunderbird Does problem go away? - If no, then cause is either: bug in Thunderbird, a setting, file or folder (something) in your Thunderbird profile, your mail provider. Please attach to the bug report or support topic a file created from Help | Troubleshooting | copy text to clipboard - If yes, the problem is gone, then (still in Windows safe mode) ... start Thunderbird normally -- If problem is still gone, then cause is likely a program loaded during windows startup. Possibilities include: antivirus software (AV), virus/malware, background downloads such as Windows or other program updates, to name a few. For AV information see https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues -- If problem came back, then cause is likely a Thunderbird add-on - eliminate them by disabling add-ons one at a time in Tools | add-ons | extensions and restart Thunderbird after disabling (if you have many add-ons, try disabling half of them at a time) - 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.