It'll (theoretically) be faster on any hardware that supports 64 bit operations, as SHA512 ends up doing fewer block operations.
This assumes you're hashing something at least 8 bytes long, and that your hash implementation is smart enough to use the 64 bit capabilities of your platform.
This assumes you're hashing something at least 8 bytes long, and that your hash implementation is smart enough to use the 64 bit capabilities of your platform.