Medium Large Transport
Air Traffic Control Developmental
Situational Awareness
Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Procedure
I was coming out of the hold and Aircraft X and Aircraft Y were the last two I was clearing back to KORD. Aircraft Y was already direct FIYER (West-southwest bound) and the remainder of the WYNDE5 arrival and had vacated 130.  Aircraft X was on a 090 heading at 140.  When Aircraft Y vacated 130 under a descent clearance (not crossing or discretionary) I assigned Aircraft X 130.  Even though both aircraft had similar performance characteristics; Aircraft Y descended very slowly while Aircraft X descended at a higher rate.  When Aircraft X; now level at 130; came within 5 miles of Aircraft Y; Aircraft Y was still at 122 and descending. Jets have a large performance envelope and can climb/descend at anywhere from 500 to several thousand feet per minute.  When stepping down one aircraft on top of another one; I should have paid attention to the rate as derived from the MODE C or just have waited until the lower aircraft had vacated the next lower altitude before descending the aircraft that was on top.  That would have ensured 1000 feet of separation.
A ZAU Controller used the vacated altitude rule and had a loss of separation due to the descent rate of one of the aircraft being different than the other.