ZJX CTLR EXPERIENCED OPERROR AT 7000 FT WHEN INCORRECTLY ANTICIPATING DSCNT RATE AND CONFLICT DEVELOPED WITH ENRTE ACFT.

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ACFT X INBOUND TO SSI. ACFT Y ENRTE TO UDG AT 7000 FT. I DSNDED ACFT X TO 8000 FT (OTHER TFC AT 7000 FT). ONCE ACFT X WAS CLR I DSNDED ACFT X TO 6000 FT (TFC AT 5000 FT). ACFT X WAS FASTER THAN ANTICIPATED AND DSNDED SLOWER THAN EXPECTED AND CONFLICT ALERT WENT OFF BTWN ACFT X AND ACFT Y. I CALLED TFC TO ACFT X. HE DID NOT HAVE ACFT Y IN SIGHT SO I TURNED TO A 280 DEG HDG. I CALLED TFC TO ACFT Y AND HE RPTED ACFT X IN SIGHT AND RPTED NO FACTOR. I ADVISED ACFT Y TO PROVIDE VISUAL BUT VERT WAS ALREADY LOST. I HAD TURNED ACFT Y TO A 080 DEG HDG AND THAT IS WHEN HE ADVISED HE WILL PROVIDE VISUAL SEPARATION. I WAS CTLING APPROX 14 ACFT AT COMBINED (R53; R54) SECTORS WITH A RADAR QUALIFIED RADAR ASSOCIATE; AND A TRACKER WHICH HAS BEEN PLUGGED IN ABOUT 5 MINS. I WAS CTLING 2 ARRS INTO JAX WHICH WAS REQUIRING SPD AND ALT CTL FOR SEQUENCING. 5 ACFT INTO SSI; BQK (LOCATED WITHIN 5 MI OF EACH OTHER) RANGING FROM C172 TO BE20 TO LR60. I HAD 2 VFR ACFT CONVERGING AT 9500 FT WHICH DID NOT SEE EACH OTHER YET (1 WAS NORDO) OTHER WANTED TO CLB TO 10500 FT FOR AVOIDANCE BUT TFC SBOUND AHEAD AT 10000 FT. I WAS GIVING VFR FLT FOLLOWING TO APPROX 6 ACFT AND NUMEROUS MORE VFR BEING DISPLAYED ON RADAR AROUND SSI. POSSIBLE REMEDIES TO PREVENT A RECURRENCE: SPLIT THE SECTORS; HAVE ARRS INTO SSI ARPT LOWER AND HANDED OFF TO SAV APCH TO PROVIDE DSCNT FROM THE N; RECOGNIZE SPD OF ARRS (I THOUGHT THE ACFT WAS SLOWER THAN ACTUALLY WAS). LIMIT VFR FLT FOLLOWING TO A MINIMUM. BETTER TFC PLAN TO ENSURE OTHER SEPARATION RATHER THAN ASSUMING VISUAL SEPARATION.