I'm sorry, but I've never heard anyone say Godot is fairly unkind to older hardware before.
Sure, yeah, if somebody is futzing around in 3D, in the Forward+ renderer, and has no idea what they are doing, yeah.
But... broadly?
How... old of hardware are you talking about?
Like, 15+ years old?
Also, SDL isn't ... a game engine.
Its... a rendering/input/output layer/library.
Sure, if you want to write your own game engine, you could use SDL... but... that's a bit much to ask of a novice indie dev, who wants to complete a 3D game that's maybe roughly as or more graphically advanced than say, Fallout New Vegas, in under what, 3, 4, 5 years?