It's a product of many cooks and their brilliant ideas and KPIs, a social network for devs and code being the most "brilliant" of them all. For day to day dev operations is something so mediocre even Gitlab looks like the golden standard compared to Github.
And no, the problem is not "Rails" or [ insert any other tech BS to deflect the real problems ].
Gitlab is anything but light, by default tends to be slow, but surprisingly fast with a good server ( nothing crazy, but big ) and caching.