How does the Nav computer calculate the RF (radius-to-fix) leg? Does it use TAS + Wind Vector or does it simply use the instantaneous ground speed?
An RF leg that is part of a charted procedure (presently [mostly] only RNP AR, but will become more widespread than that) is fixed in the nav database with an arc center point and a radius value. The criteria used to design the RF legs in RNP AR have TAS values based on altitude and adverse wind assumptions agreed to by the FAA and industry. The idea is to not exceed a 25 degree bank under the worst circumstances.
As to RF legs computed "on the fly" by the avionics for a course change at a fly-by waypoint, I defer to the avionics gurus, and it could vary from vendor to vendor. Those "on the fly" RF legs certainly do vary with ground speed.