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I know you're asking about RF lenses but if you really want speed then forget about RF. I have owned and shot both the v2 and current apo asph summicrons. The v2 isn't particularly sharp wide open at any distance and the apo is not sharp in the closer range where you would dhoot head shots. I really dislike minf for that use otherwide at distances greater then 8-10 feet it's very good. I also own and shoot the v1 elmarit and the v2 thin tele-elmarit. The v1 elmarit is very good at all apertures and all distances. The Tele flares like crazy even with the proper hood and isn't super sharp untill you get to 5.6. The Nikkor 105 is exceptional by any standard. My 75 1.4 summilux is very fine but even on a .85x MP at close distance it's tough to focus accurately. RF's aren't great portrait tools IMO. they excell for normal to wide lenses and are marginal for longer.
If you really want a fast lens for shallow dof then forget RF's. The Nikkor 85 1.8 AF, 85 1.4 and the canon 85 1.2 are the winning ticket for speed and super sharpness. I have the 85 1.2 and it's one of the sharpest lenses I've ever owned and the OOF in a head shot at 1.2 is fantastic. The canon 135 f2 is in the same class and even a little sharper.
If you really want a fast lens for shallow dof then forget RF's. The Nikkor 85 1.8 AF, 85 1.4 and the canon 85 1.2 are the winning ticket for speed and super sharpness. I have the 85 1.2 and it's one of the sharpest lenses I've ever owned and the OOF in a head shot at 1.2 is fantastic. The canon 135 f2 is in the same class and even a little sharper.