MIA APCH CTLR LOSES STANDARD SEPARATION WHEN VECTORING 2 ACR JETS TO THE MIA RWYS 9 FINAL APCH COURSE.

Narrative

I WAS WORKING THE FEEDER FINAL POS AT MIA TRACON. ACFT X WAS ON L DOWNWIND FOR RWY 9L. ACFT Y WAS ESTABLISHED ON THE RWY 9R LOC. ACFT Y WAS AT 3000 FT. ACFT X WAS HDG 260 DEGS WHEN I TURNED HIM TO HDG 180 DEGS. HE APPEARED TO BE TURNING UNUSUALLY SLOWLY TO THE 180 DEG HDG; SO I THEN ISSUED THE HDG 160 DEGS AND THEN 120 DEGS TO JOIN THE RWY 9L LOC. DURING THOSE TURNS; I QUESTIONED ACFT X ON HIS RATE OF TURN AND WHETHER HE WAS EVEN TURNING AT ONE POINT. ACFT X APPARENTLY HAD HELD THE 180 DEG HDG FOR 12-16 SECONDS EVEN THOUGH HE WAS ISSUED THE 160 DEG AND THE 120 DEG HDG BEFORE HE EVER REACHED HIS 180 DEG HDG. I EXCHANGED TFC BTWN BOTH ACFT. I ASKED ACFT Y IF HE COULD MAINTAIN VISUAL SEPARATION WITH THE ACFT X TAKING THE WIDE TURN TO THE PARALLEL FINAL. ACFT Y SAID HE WOULD MAINTAIN VISUAL SEPARATION. I HAD DSNDED ACFT X TO 1500 FT ON THE DOWNWIND. THEIR CLOSEST PROX WAS ABOUT 2 MI AND 800 FT. I CLRED BOTH ACFT FOR ILS APCHS THEN SWITCHED THEM TO THE TWR. I BELIEVE THE LOSS OF STANDARD SEPARATION OCCURRED BECAUSE THE PLT OF ACFT X FAILED TO COMPLY WITH AN ATC INSTRUCTION IN A TIMELY MANNER.