I don't know, to tell the honest truth. I just remember you saying that Preview was slow as molasses ported to QtWebEngine when you were experimenting. I doubt it's gotten much better with 5.9.
I'm able to get good Windows builds of the new (github.com/annulen) QtWebKit, now, so I'm going to experiment with the nightly snapshots to see if it's gotten any better since the last release. I submitted a bugfix (through Konstantine Tokarev)
upstream at WebKit to fix the
local-media auto-play issue, so I'm curious to see if it alleviates any of QtWebkit (and hence, Sigil's) Windows Multimedia Foundation woes with multiple video/audio elements in one page. It's just finding the time.