TFC SEPARATION PROB AT DEN CAUSES AN ACR CREW TO SLOW BELOW THE ATC ASSIGNED SPD.

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WE WERE UNABLE TO COMPLY WITH ATC SPD CLRNC. ON VECTORS TO INTERCEPT RWY 16 LOC; DEN APCH SLOWED US TO 170 KTS AND ASKED IF WE HAD THE FIELD IN SIGHT. WE RPTED IT IN SIGHT; SO THEY CLRED US FOR A VISUAL APCH TO RWY 16; MAINTAIN 190 KTS. AS WE SPD BACK UP TO 190 KTS; WE SAW TFC WHO WE THOUGHT WAS FOR RWY 17R. THE TFC THEN DID A HUGE S-TURN BACK TO RWY 16. WE THOUGHT WE WERE WAY TOO CLOSE (THE TFC WAS NEVER CALLED TO US); WITH TCASII RANGE 3 MI AND CLOSING FAST; SO WE ADVISED APCH THAT WE WERE SLOWING FOR THE TFC IN FRONT OF US. DEN APCH ASKED US TO MAINTAIN 170 KTS TO LEETS AND CONTACT TWR. THERE WAS CONVECTIVE WX IN THE AREA AND A POSSIBILITY OF LLWS (THOUGH NONE HAD BEEN RPTED YET). AS WE SLOWED TO 170 KTS; DISTANCE WAS STILL CLOSING AND WE COULD SEE THAT IT WAS AN ACR Y ACFT. WE CONTACTED TWR AND ADVISED THEM WE WERE SLOWING FOR TFC IN FRONT OF US (WE WERE 3 MI FROM LEETS; AND 170 KTS WOULD BRING US TOO CLOSE TO ACFT IN FRONT OF US. TWR RESPONDED WITH 'MAINTAIN 170 KTS TO LEETS AS PREVIOUSLY INSTRUCTED.' SINCE WE WERE ONLY 1 MI FROM LEETS AT 150 KTS (3 MI FROM TFC) WE RESPONDED 'UNABLE' AND CONTINUED. WE HAD ADVISED THEM IN PLENTY OF TIME TO SLOW ACFT BEHIND US TO CORRECT THEIR SEQUENCING MISTAKE.