Okay, I took a look at your epub and it said the following in its OPF:
Quote:
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<dc:language>en</dc:language>
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and it has the following in its cover.xml file (and no other place).
And most importantly it has 7 plus chapters all squeezed into one huge html file with over 6300 lines to parse.
That one huge file (which is equivalent to putting almost the entire book in one file) is the reason for Sigil's "slowness".
There are already sigil_split_markers in the text (too many of them so it will end up splitting things in the middle of chapters too).
To "speed up", you should:
1. first go to the View menu and turn off Preview Mode (which is taking all of the time as it is trying to load and render the entire book all at once in one html file). After that, performance should "improve".
2. Next you should use Sigil to rename the "cover.xml" file to be called "cover.html" as an xml file can hold anything and it will prevent Sigil from running split as it will not know how to handle that file.
3. Next after removing the extra sigil_split_markers, you should then use Sigil's Split at Markers menu to split that huge file into the 7 chapters.
4. Then you can turn back on Preview using the Tools menu.
Finally, I could not recreate your spellcheck issues at all. I simply went to Sigil and changed my Primary SpellChecking dictionary to English - Great Britain and then added the following line to the first html file: "My favourite color is red." and the spellchecker detected "favourite" as correct and "color" as incorrect as expected.
So your "slowness" is due to a very poorly built epub file. And I can not recreate your spellcheck issues at all.