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The bug where you directly import an html file as its own epub in Sigil, and therefore the required ncx did not exist (under epub2), has been fixed in master and will be in the next Sigil release.
If you have trouble because an html file uses huge run-on lines with no line breaks (that slows any line based editor with syntax highlighting to a crawl) simply run Sigil's mend on it and proper linefeeds will be inserted to make the line editing capability of Sigil work better and make the file much more editable and readable.
Large numbers of short lines should not be a problem for Sigil given you have enough memory and reasonable performance on the machine you use to live edit it.
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