a while a ago I looked into fast-hopping using a (possibly modified) B210. Basically UHD is issuing a reconfiguration of the AD9361 for each tune which will not only wait for the LO to settle, but also will due DC-offset and IQ balance calibration in the AD9361.
For fast hopping there is a special mode in the AD9361 manual where you can basically save the settings after calibration either in a special set of registers (limited space) or supply them on the fly so no calibration is carried out. Using this it would be possible to possibly to sub-ms tuning. (I don't recall the details) In the end I did not implement this but it would be definitely feasible (would require a bit of work). Maybe someone else has some more details on this (I think there is something in the archives on this).
Cheers Andrej -- Sent from my Android device with K-9 Mail. Please excuse my brevity.