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https://www.sciencealert.com/zooming-black-holes-can-reach-10-the-speed-of-light-scientists-say
https://www.sciencealert.com/zooming-black-holes-can-reach-10-the-speed-of-light-scientists-say
Probably relative to the CMB (the frame of reference where there is no redshift or blueshift bias in any direction).