Which speed shall be used on high speed taxiway?
According to the ICAO manual on Aerodrome Standards (based on Annex 14) (link) - maximum 35 knots for code 1 and 2 runways, and maximum 50 knots for code 3 and 4 runways.

Since you are moving on the ground, it makes sense to use your ground speed as reference.

I imagine most aircraft operators have procedures for using rapid exit taxiways in their standard operating manuals. As a result, the actual speed used by air crews is likely to vary depending on which company they fly for.

IFALPA recommends that you slow down to normal taxi speed on the runway, and then use rapid exit taxiways with normal taxi speed (as opposed to the very low speed you would need to use to turn off the runway using a pendicular taxiway). (link)

From the ICAO manual:


  3.8.15
  
  A rapid exit taxiway shall be designed with a radius of turn-off curve of at least: 
  
  
  550 m where the code number is 3 or 4; and 
  275 m where the code number is 1 or 2; 
  
  
  to enable exit speeds under wet conditions of: 
  
  
  93 km/h where the code number is 3 or 4; and 
  65 km/h where the code number is 1 or 2.
  


Note: you can find the definitions of the code numbers (1, 2, 3, 4) on page 17 of the manual to which I linked above.