The performance is the same because the core loop in both cases is identical:

    loop:
        vaddsd	(%rcx), %xmm0, %xmm0
        addq	$8, %rcx
        addq	$-1, %rax
        jne	loop

There's more setup required for offset arrays, but otherwise there's no difference. I'm not sure why arrays having an offset would be any more work: array indexing in general involves adding an index to a base pointer; having an offset effectively just changes what the base pointer value is.