If b is a 64-element BitArray, then b.chunks[1] is the same data as an Int64, but in bit-reversed order. There are various algorithms for efficiently bit-reversing back to regular order; I’m not sure which one is the fastest, but you can definitely do better than going bit-by-bit.
(But if this conversion is really performance-critical for you then a BitArray may not be the best choice of data structure.)