Hi Veloo
The term 'High Speed Climb' refers not to the actual rate of climb, but the airspeed. Below FL100 there is generally a 250kts speed restriction. So when this is ATC restriction is removed, the crew can increase to their optimum climb speed which varies greatly depending upon the aircraft, load and the parameters dictated by the operator.
There is an impact upon the rate of climb but there is no 'rule of thumb' as to by what. The rate is more a function of the aircraft's potential performance and loading.
Hope this helps