B757 CREW INBOUND TO ORD ON RWY 22R ILS FINAL AT 170 KTS OBSERVED AN ACFT TURN IN 3 MI AHEAD ON THE SAME FINAL AND S-TURN.

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WE WERE VECTORED ON A 15 MI FINAL TO RWY 22R AT ORD; AT NIGHT IN VMC. WE RECEIVED A CLRNC FOR THE ILS RWY 22R APCH AT 170 KT SPD. AT 10 MI FROM RIDGE; THE FO AS PF AND MYSELF SAW WHITE POS TRAILING EDGE LIGHTS CUTTING ACROSS THE RWY 22R FINAL IN A ZIG-ZAG PATTERN AT OUR ALT IN FRONT OF US. TCASII CONFIRMED THE TARGET AND ALT. HE SHOWED UP AS A TA AT ABOUT 3 MI IN FRONT OF US. NO MENTION OF THE TFC FROM ORD TRACON. THE ACFT APPEARED TO BE S-TURNING ON THE FINAL AND FROM THE VISUAL PICTURE AND CONFIRMATION ON TCASII WE WERE CLOSING ON HIM. THE FINAL APCH CTLR CHANGED AT THIS POINT FROM A MALE TO A FEMALE. WE WERE STABILIZED AND AT 170 KIAS. WHEN SPACING WENT BELOW 2.5 MI; WITH THE TFC STILL S-TURNING; IT STILL APPEARED THAT WE WERE CLOSING; THE FO SLOWED TO 155 KIAS. FREQ WAS BUSY; BUT AS SOON AS THERE WAS A BREAK I RELAYED THAT WE SLOWED TO 155 KTS; WE WERE JUST INSIDE OF 4 MI FROM RIDGE. THE CTLR BECAME BELLIGERENT AND STARTED DESCRIBING THAT SHE WAS IN CHARGE OF SEPARATION NOT US AND THAT WE NEEDED TO BE AT 170 KTS; THE SPACING WAS ADEQUATE AND THE SPDS MATCHED. BY THIS TIME WE WERE ABLE TO GET 2.5 MI SEPARATION ON THE HSI SCALE AND THE SPDS WERE MATCHED BECAUSE WE HAD SLOWED. SINCE WE HAD THE CORRECT SPACING AND THE VISUAL PICTURE LOOKED CORRECT; WE PICKED THE SPD BACK UP TO 170 KTS. TO MY KNOWLEDGE THIS SPD ASSIGNMENT IS AN ATC INSTRUCTION; NOT AN ATC CLRNC; AND WE INDICATED TO ATC THAT WE NEEDED TO FLY LESS THAN THE ASSIGNED SPD DUE TO THE ACFT MANEUVERING IN FRONT OF US. FROM MY POS; BOTH VISUALLY AND CONFIRMED ON TCASII; ATC WAS PUTTING OUR FLT AT RISK DURING A CRITICAL PHASE OF THE APCH; SO I ELECTED TO SLOW AND INFORMED ATC. CONTRIBUTING FACTOR COULD HAVE BEEN THE CTLR CHANGE DURING A MINIMUM SPACING EVENT ON THE FINAL WITH THE ACFT IN FRONT OF US TRYING TO STABILIZE BY S-TURNING ON THE FINAL; WHICH CAUSED THE SPACE INTERVAL TO DECREASE AS HE COVERED MORE GND. THE TRACON CTLR SHOULD UNDERSTAND THE CAPT IS CHARGED WITH THE SAFETY OF THE FLT AND IN THIS CASE; I FELT THE SAFETY MARGIN WAS BEING COMPROMISED AND MADE THE DECISION TO GET THE SPACING BACK AND INFORM ATC.