Ultrasound researcher working on fast microvascular imaging here. Depends on the datarates, you can generally get pretty good blood flow data pretty fast, with <.5 seconds per slice if you do it with appropriate algorithms. A lot of this depends on motion though as you said, as the issue is generally getting a high-enough SNR (blood flow is generally 30-40 dB below tissue in energy, except for the biggest of vessels). Generally, higher frequencies have less of a separation between blood and tissue, but they have issue with attenuation. But I think with enough SNR (and with their element count that may indeed be possible), they could get pretty good blood flow data across the whole body.