The alt_bn128 elliptic pairing optimizations, such as the use of the specific generator points, are not being used. Enabling them requires compiling the libff dependency with PERFORMANCE=On, rather than off as done currently: https://github.com/ethereum/cpp-ethereum/blob/bbfa6e004f2c852e11b60a452b2fec6a36b370e9/cmake/ProjectSnark.cmake#L17