Decompilation is just that - this code hasn’t been ported to new libraries or platforms yet. This means it targets N64 and is still designed within boundaries of that platform.

In terms of performance it’s hard to tell how optimised decompiled code is unless you can compare it with the original using the same compiler Nintendo did. Compilers these days create much more optimised machine code than they did in the 90s so you’d likely get significant boost from compiler alone.