Suppose an incompetent developer would push an arbitrary change, miners will think twice before adopting it.
Again this has little to do with size and mostly it is about decentralization.
In a centralized altcoin a change is "pushed" however in a decentralized bitcoin the changes are "proposed" and that's just the start. After that the proposal is discussed and usually get improved or changed then implemented and then starts being tested to find weaknesses and improve some more or maybe even reject at this point.
After all of this is done, it is put forth for users to accept (nodes upgrade to the new version and miners signal for the new change). If an incompetent developer proposes any bad change it would be rejected at the very beginning. This process has been in bitcoin from the beginning when bitcoin wasn't worth anything. Even Satoshi didn't just "push" changes, they were discussed and reviewed in that small community back then with Hal Finney et al.
BTW this is why OP is calling this process "beyond slow".