I put this together to show the speed of Redshift. This isn't meant to compare directly with Mantra, but to give a rough estimate of GPU rendering. I would guess that any other CPU renderer would take about the same amount of time. You'll also notice that they look slightly different. I did my absolute best to match the shader settings although they do look different. The lighting and environment are the exact same. But again, they are different renderers so go figure.
My hardware setup for Redshift is 3 GeForce GTX 1080Ti's and my CPU setup is an Intel i7 5960X running at 3.75GHz with 64Gigs of DDR4 Quad Channel RAM running at 3000MHz.