You should be able to compile the decompiled code to run on the original hardware so it'd be basically 1:1 performance even if the code itself isn't. It's just kinda pointless to do. A real world example...Kaze sort of is? But at this point the Mario 64 game engine is more his code than Nintendo since he's modified it so extensively for his upcoming game.

With emulation you're emulating the entire console's hardware first at full speed ideally. Not just the game. Normally that will always be more demanding to run than native code.

Similar spec hardware to the N64 kind of depends. It's more about what's done for each port that uses the decompiled code. Like for desktop people will use desktop API of course. If it's say Vulkan, you'd then require a GPU that supports whatever the developer specifies as the minimum version supported. Certain CPU instructions, whatever.

Mario 64 has decomp ports running on Raspberry Pi 3 and Nintendo DS. There may be weaker hardware ports but those are the ones I knew about as of writing.