What is the impact of thermal Anti-Ice on descent performance?
When turning on the Anti-Ice, the demand for bleed air increases and the EEC (Electronic Engine Control) will increase the target N1 to provide enough pressure by injecting more fuel. From the Boeing 737 NG FCOMv2 (7.20.5 Engines, APU - Engine System Description, emphasis mine):


  In the normal mode, the EEC uses sensed flight conditions and bleed air demand
  to calculate N1 thrust ratings. The EEC compares commanded N1 to actual N1
  and adjusts fuel flow to achieve the commanded N1.


This impacts the descent performance as explained in the Boeing 737 NG FCTM (4.24 Climb, Cruise, Descent and Holding):


  The use of anti-ice and the increased thrust required increases the descent
  distance. Therefore, proper descent planning is necessary to arrive at the initial
  approach fix at the correct altitude, speed, and configuration. The anticipated
  anti-ice use altitude should be entered on the DESCENT FORECAST page to
  assist the FMC in computing a more accurate descent profile.


Note: the above quote is for the 737 NG models. The older 737 classic series does not increase thrust:


  Since all engines have been manufactured or modified to use a single idle speed
  in flight, the use of TAI does not increase thrust as it does in other models.