Unbiased actually isn't slow - Arnold has proved this - what's slow is using hundreds of bounces per path and bi-directional path tracing (like Maxwell, Indigo,

Unbiased actually isn't slow - Arnold has proved this - what's slow is using hundreds of bounces per path and bi-directional path tracing (like Maxwell, Indigo, LuxRender) which takes a lot longer. Biased generally means it's interpolated with a point or irradiance cache.