Incredibly strange. We haven’t updated it, and unless the extension fetches things from elsewhere (ours do not) I don’t think it can do what you are describing? Could be a browser fix?
I can reproduce the loading time issue. Interestingly, when I convert your WARC to a WACZ with pywacz it does not result in long initial load times which would suggest this could be a problem in ArchiveWebpage.
It’s also worth noting that (while it’s quite hidden) you can export WARC files from ArchiveWebpage — though they won’t have a pages index in ReplayWebpage. WARC files don’t seem to experience this issue.
This option can be found in the pages tab under the download dropdown. Eventually this will be added as an option on the main archived item list, it’s on the always growing list of small things to fix 
So no immediate solutions… We’re in the middle of refactoring both ArchiveWebpage and ReplayWebpage so any updates will have to wait until that process is complete. I’ll pass along your experience to the rest of the team. Thanks again for the files and again for a detailed video!