[DiapDealer]

With just the Book Browser and Code View widgets open (and with spell-checking and tag highlighting disabled) the same massive merge takes 1.5 minutes in 1.5.1 and just a hair less than 20 seconds after your changes. That's great!

Unfortunately, the stuttering and freezing continues (as well as the delay when trying to launch/render the preferences dialog afterward with little change that I can detect). But I'm beginning to detect a pattern that can help eliminate the problem (temporarily at least). The issue seems to be that the Code View tab with the new giant html file (from the merge) has the focus. If I can get the focus on the Book Browser widget (by single-clicking one of the files in the tree and waiting for the inevitable busy cursor to stop spinning) then the various menus (including launching the Preferences dialog) are very responsive and don't suffer from the "freezups"... even though the giant file is still the active tab showing in the central widget.

I know there's no way to keep monolithic html files from slowing down Sigil in general when editing, I'm just wishing there might be a way to mitigate the effect if the monolithic file's tab is open (and active) but is not being actively edited, scrolled, clicked on, or rendered (Preview closed).

P.S. I also think the fact that the drop down menus (as well as the Preferences widget) can partially oclude the Code View tab of the giant html file might be causing signals to fire and reparsing to happen. Trying to launch Preferences on my laptop with the monolithic file open actually seems to cause a loop of some kind that prevents the widget from ever fully rendering. If I shrink the Code View tab so that none of the dropdowns touch Code View (and make sure the Preferences dialog is positioned to not open over top of the Code View Tab), the freezing menus and non-rendering Preferences don't seem to present nearly as much of an issue.