[KevinH]

Even old AV can see new installs as a threat. I would try unplugging your internet connection (to be perfectly safe with no AV during this test), doing a safe reboot to prevent issues from system extensions and other non-official system software, turning off AV, then testing that epub. If no slowdown, then the issue is either your AV or some system level extension. You can then turn back on AV, plug back in internet, and reboot.

If AV there may be a way to tell it to leave Sigil alone. Many AVs constantly check new files generated in /tmp and lock the files during scans. Since Sigil uses the system /tmp directory for processing books, this causes a huge slowdown. There is a setting in Sigil to specify a different (private to Sigil) temp directory, so you might want to try that.

For general slowdowns, interference from AV or installed third party system extensions is almost always the culprit.

Also please supply the test epub I asked asked about so that we can track down what is causing the spellcheck issues so we can fix that for the next release.