ARTCC Controller experienced a loss of separation between two Air Carrier arrivals during descent while trying to fine tune the speed assignments.

Narrative

I was working two arrivals. The DH8 was slightly behind a B737 arrival. I placed a halo on the DH8 and descended him on initial contact after accepting a hand off on a B737 on the same route. I expected the B737 would eventually be far enough in front of the DH8 and would be able to keep both descending to meet the arrival restrictions to approach. When the B737 was outside the halo and still slightly faster; I gave both aircraft the crossing restriction. After the clearances; the next RADAR hit activated the Conflict Alert (CA). The B737 was inside the halo. Checked speed readouts; B737 still faster. Next RADAR hit put the B737 outside the halo; B737 still faster. Next RADAR hit put B737 inside the halo. [I] questioned speed of the B737; ground speed of the B737 still faster. [I] waited one more RADAR hit; B737 still faster; but CA still activating. [I] initiated a 10 degree turn to the right for the DH8. Then another turn and speed restriction for the DH8. Recommendation: I relied on RADAR data and ground speed readouts. Considered aircraft characteristics; two aircraft on the same route; RADAR shows faster in front the whole time. Recommend a review to see the accuracy of RADAR data.