looks like the MOOV atom isn’t at the beginning of the file.
I used ffmpeg to just relocate that (no other encoding) and then a binary compare (using HexFiend) and a quick test seemed to show that Safari was playing the video sooner
./ffmpeg -i top.mp4 -codec copy -movflags faststart top-fs.mp4
(caveat being that even though I cleared browser cache I didn’t do things like bounce my test server or time things too accurately)
FWIW I find ffmpeg to be a good solution, and especially for background video you’ll want to play around with parameters to optimize for your use-case