It is called Compacting Folders and has nothing to do with compressing files to save space as in Zipping files. Read this. https://support.mozilla.org/en-US/kb/compacting-folders

Thank you for the response Airmail. Yea i noticed the difference from compressing and compacting after I put the issue in.

So am I reading right that there is no way around compacting? Also If folders are compacted can it be undone?

No it cannot be undone. It actually removes messages you have marked for deletion. Why would you want to undo that? And yes, compacting is a normal, necessary part of folder maintenance. Poorly named but necessary. Start your computer in safe mode with networking. This eliminates your antivirus software and other functions. Is Thunderbird faster in this mode. The AV is the usual cause of slow operation.

Again thanks for the quick reply. There is no AV on the machine. All was running great until I said yes to compacting. Then it slowed way down. Whenever I need to move mail from one folder to the next (within the same account) it takes a long time.

When you start your computer in safe mode report your results.

maf4aja,

What are your results??

Compact doesn't cause slowness, after the compact is finished.