Your quick look is insufficient. There are custom implementations of bit setting and clearing operations that use GMP primitives instead of the Data.Bits defaults.
On Sun, 19 Jun 2016, David Feuer wrote:
I know that Sergei Lebedev did quite a bit of work (for the bitset package) optimizing certain Integer bit operations. That code doesn't work with the new Integer, but it can probably be adapted.
>From a quick look, it simply gives names of set operation to existing Data.Bits methods on Integer. I cannot see much custom optimizations.