I can't speak to the last few years since I left, but over the many years I was there the git storage layer was almost never the core issue - it was well designed by infrastructure-minded nerds that leveraged and improved git and replicated it really well across multiple nodes.

What always struggled was the richness of the Rails monolith itself and its backing MySQL databases - the expectation that everything links to everything throughout the product (think: issue cross references across orgs that only appear if you're able to access the remote repo, and other things like that).

Those details appear richly everywhere you look, and the combination of that with a general lack of understanding and/or focus on performance (shipping big features is hard, shipping them with performance at scale is MUCH harder), compounded by Ruby being an easy language to get performance wrong in (object count really hurts, and it makes it very easy to create many) leaves every feature adding to the performance problem, and makes it daunting/impossible to make fast once it's slow.

There was a full on year or more of making GitHub fast while I was there that just couldn't gain enough momentum to make enough of a dent to make it better. I remember finding and fixing a N^3 (or maybe it was N^4? something bad) in the home page activity feed - the worst thing I found but gives an idea. IMO it would need a fresh view of how to keep interfaces simple and how to design the data layers performantly - not adding every bell and whistle to every screen.

I hope someone at GitHub realises they are about to lose everything that was hard earned by early GitHub - it once was a site people (myself included) looked up to for ideal availability, responsible releases, data driven improvement - but no more it seems :(