import difflib


def record_diff(old, new):
    """
    Generate a human-readable diff of two performance records.
    """
    return '\n'.join(difflib.ndiff(
        ['%s: %s' % (k, v) for op in old for k, v in op.items()],
        ['%s: %s' % (k, v) for op in new for k, v in op.items()],
    ))