Instead of (or in addition to) testing or even buying different softwares, you might extract the files of one of your converted epubs and try to figure out what causes the problem (slow highlighting). Epub files are zip files, you can simply unzip them. The text of the book is contained in html or xhtml files. Try to find something that looks somehow suspicious. For instance, if it is a long book and the whole text is contained in a single html file, this might perhaps already cause a performance problem. Or maybe you find that there is strange formatting, say every single letter has its own format tags, or every line is formatted as a table, or something of this kind. If you cannot do it yourself and the book is not restricted by copyright, you can post it here and maybe someone will look into it.
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