ARTCC CTLR HAD LESS THAN STANDARD SEPARATION WHILE DESCENDING AN MLG THROUGH ALT OF LGT.

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MLG Y WAS HANDED OFF TO ME DSNDING FROM FL240 TO 150. LGT X WAS HANDED OFF TO ME FROM APCH CTL CLBING TO 14000'. USING THE COMPUTER GND SPDS AND THE J CIRCLE (5 MI CIRCLE USED AT CENTER FOR SEP). I OBSERVED MLG Y AT 350 KTS AND LGT X AT 330 KTS. LGT X WAS INTRAIL AND TO THE LEFT OF MLG Y BE A MEASURED 7 MI. I CLBED LGT X TO HIS REQUESTED ALT OF 16000' AND HE LEVELED OFF. SPDS STILL REMAINED CONSTANT FOR BOTH. ABOUT 1 MIN LATER I DSNDED MLG Y TO 11000' AS HE WAS LNDG AT BDL ARPT; WHICH WAS ABOUT ANOTHER 50 MI. AS MLG Y WENT BELOW FL180 DSNDING TO 11000' HIS GND SPD DROPPED TO 330 KTS. AT ABOUT THIS TIME LGT X INCREASED HIS GND SPD TO 390 KTS. I TURNED BOTH ACFT 260 DEGS HDG TO MLG Y AND 240 DEG HDG TO LGT X. MINIMUM SEP WENT TO 4.68 MI AS ALTS PASSED AND THEN INCREASED BECAUSE OF THE HDGS. NO EVASIVE ACTION WAS NEEDED. HAD I KNOWN OF THE SPD CHANGES I WOULD HAVE ASSIGNED INDICATED SPDS TO BOTH ACFT. THE CTLRS' HANDBOOK AND THE AIRMEN'S INFO MANUAL BOTH STATE THAT THE PLT IS REQUIRED TO TELL ATC OF SPD CHANGES OF MORE THAN 10 KTS. ALSO LGT X COMPANY HAS ITS OWN ORDER OUT FOR THEIR ACFT TO FLY A 300 IAS FOR FUEL SAVINGS. LGT X HAD TO BE DOING AROUND 350 IAS.