I'm gonna guess it's the authorities. It's usually them. Google is presumably making money off of it and has little incentive to do it, especially piecemeal, while Russia has a history of censoring and often throttling internet access.
Also, Radio Free Europe/Radio Liberty, which is, I should note, funded by the US government, claims that's what's happening [https://www.rferl.org/a/youtube-russia-throttling-tech-censo...]. So it seems quite likely that it is, in fact, Russia's government behind this.
