The Tl;dr version is that Street Fighter 5 launched with a poorly implemented version of 'rollback netcode' 4 years ago, giving online matches issues they don't need to have.
Developers who have worked on functioning rollback based fighting games, like Keits who worked on Killer Instinct, among others, have gone on record saying that "it wouldn't take long for Capcom to fix this."
People suspected that SFV has a bug that causes people's 'clocks' to de-sync.
A fan on Reddit, who doesn't have access to the source code like Capcom does, created a patch for the PC version in 2 days that fixes the de-syncing.
It would take Capcom, who do have access to the source code, less time to produce the same results.
Of course they'd have to do proper testing and such, but I find it pretty remarkable that something that took a fan 2 days hasn't been done by Capcom in 4 years.
The reason why Capcom launched the game this way, and why they haven't fixed it since are touched on in the videos.
Rollback netcode has been an especially hot topic in the FGC for the past few months now.
Essentially, the majority of Japanese fighting games use delay based netcode instead. Because 1.) Japanese dev are not as familiar with rollback, and 2.) because Japan is a smaller country with better internet infrastructure, so during their testing they get good results between players of opposite ends of the country, and think the game is fine and ship it worldwide.
SFV is one of the few prominent Japanese fighting games to use rollback. (The HD re-release of SF3 Third Strike also used rollback. But it was already an old game at that point.) But because it was implemented poorly in SFV, it gave some people, and perhaps the SFV devs, the impression that rollback isn't good.
That's part of why this fan made patch is important. Hopefully it catches Capcom's attention.
A sudden maintenance was announced for Jan 14.
If it's about this, then Capcom may either disable the patch, or (less likely, but hopefully) create their own and distribute it for all platforms.