Since it doesn't appear to be related to Sigil's Clipboard diddling, I have to take it as a good sign that calibre's editor can select text in a QPlainTextEdit widget (with line numbers and syntax highlighting) with no noticeable lag on the same machines that lag using Sigil (with comparable recent versions of Qt5). It's the optimist in me.
Should we revert the change to dataChanged() back to changed(QClipboard::Mode) in master?