[DiapDealer]

Using the tests described (and the epub provided) by @selenia on my i3 8Gb laptop, I am able to experience a fairly noticeable difference in speed between 1.4.3 and 1.5.0. The times may not be exactly the same, but everything coincides with their experiments.

It doesn't seem to be dependent upon the tag highlighting or spellcheck settings, turning those off/on seems to make no difference.

What I DO notice right away when opening the file in question (or just about any file in any epub, really) is that starting with Sigil 1.5.1, Preview seems to be slowly "double-pumping" when opening a file: I see the file load, then I see some css apply, then both of those things happen again (when opening any of the epub's files). I see this "double-pump" occasionally on older versions of Sigil as well, but it happens much more quickly, so it's not as pronounced.

The bottom line is that Preview seems to be loading/rendering more slowly in 1.5.1 compared to older versions of Sigil (at least on Windows with hardware that's not supercharged). The good news is that closing Preview makes these search experiments happen pretty much the same speed across all the versions of Sigil tested (1.3.0, 1.4.3, 1.5.1, and a build from github that's only missing the last two commits).

I still experience the same, much slower, double-load of Preview using the master branch of Sigil's github repository (2 commits shy of HEAD), but the new asynchronous loading of Preview means that it no longer affects the search test speeds.

I'm going to try to see if I can grab some video of the double Preview loading going on. But I suck at that sort of thing, so it may take me a bit.

EDIT: Added zip archive of two videos. One for 1.5.1 and one for 1.3.0. MR doesn't allow mp4 file extensions, and I'm not going to take the time to re-encode the videos. Sorry. Just me clicking on the "Find" button with epub in question.