FWiW, I tried extending a vertical selection with the keyboard (indeed, gets slow over large number of lines) and using the mouse (shift+ctrl+click) which looks reasonable (maybe because it's a single shot). Anyway, this would be a Scintilla issue, so it'd be better to report to them. And as a mean to "select a large number of lines" at once, you've got what I believe is a reasonable answer with the mouse solution :)
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