Since the SSAO fragment shader is a building block, it can be easily exchanged by other techniques, which I am considering (not replacing but support for more than one technique). So, other, faster techniques might be possible in the future.

Another way to speed up SSAO in general (independent from the used algorithm) is re-projection of previously sampled AO terms and Monte Carlo-integration over time. This way less samples are taken at each discrete time step with a higher quality for samples which have been seen in the past x frames.