[KevinH]

I tried with Sigil-1.5.1 stock for macOS and I can *not* reproduce this either even with Preview open and HighLight Misspelled words turned on in Sigil Preferences SpellCheck.

Now all of my testing was done after opening the test epub and saying yes to removing the unmanifested file epub/onix.xml and after accepting the to fix the files since they are not well-formed (they are missing doctypes).

Chapter 12 opens almost instantaneously. So does starting with title and nothing else open in tabs and searching for cocaine-bottle.

So whatever is slowing down your machine is very much specific to your platform or system.

So something is making it very slow to open a new file based on your timings.

Since Sigil-1.3.0 and Sigil-1.5.1 share most of the same Qt and Python, that can not be the cause of the problem.

My guess is you have a Windows Anti-virus program that is watching for every new file created in the system temp folder (which is what Sigil does when you open an epub) and scanning it before allowing it to be opened. The Anti-virus program probably accepts Sigil-1.3.0 as okay and exempts it but for some reason has not done the same for Sigil-1.5.1.

Have you tried telling your anti-virus program to exempt Sigil-1.5.1 from its scanning?