start troubleshooting by restarting your computer in safe mode with networking and compacting. If that works, look for what on your system is loading and turning the compact experience into something less than optimal. Common problem children in this realm are; * Hard disk is actually about full. Thunderbird can only compact if there is enough room to a copy of the data being compacted. So an 8Gb gmail inbox will require more than 8Gb free on the disk. This is in addition to the normal requirements of windows which should be considered around 5% of the total space. * Antivirus product scanning the profile folder and causing file access and input / output to move glacially slow. We recommend that all anti virus scanning in the profile folder be turned off via exceptions in the antivirus product. * The profile is stored somewhere other than a local drive. Be it a local NAS or some cloud location or a server share. All are glacially slow compared to a local file access. * Streaming backups, or the location being synced to say a google drive can cause this issue as well.