ACR TAXI CLRNC CHANGED ON ROLLOUT. CLRNC CONFUSING. CROSSED RWY WITH ACFT ON FINAL.

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ORD TWR CLRED ACR TO LAND ON RWY 4R (ROLL TO THE END.) WE ACKNOWLEDGED DURING LNDG ROLLOUT; TWR ASKED IF WE COULD 'MAKE THE HIGH SPD.' WE REPLIED WE COULD AND DID. TWR'S RESPONSE WAS 'MAKE THE HIGH SPD AND KEEP IT ROLLING.' ON CONTACT WITH GND; HIS REPLY WAS 'IT'S WIDE OPEN...CARGO AND INNER.' WE ASSUMED THAT WE WERE CLRED TO GATE VIA THE CARGO AND INNER TAXIWAYS. AS WE WERE STILL TAXIING AT A HIGHER THAN NORMAL SPD (PER THE TWR'S INSTRUCTIONS - KEEP IT ROLLING.) I LOOKED L TO SEE OTHER TFC (ON THE VARIOUS TAXIWAYS; HOLDING FOR CLRNC TO GATES). AT THIS TIME WE WERE JUST MOVING ACROSS THE END OF RWY 9R. IT WAS ALSO AT THIS TIME THAT I NOTICED TFC ON VERY SHORT FINAL TO 9R. I APPLIED SUFFICIENT PWR TO PROMPTLY CLR RWY 9R. IT IS MY OPINION THAT WE WERE WELL CLR OF THE RWY BEFORE THE OTHER ACFT LANDED. AS WE APCHED THE CARGO TAXIWAY THE GND CTLR ASKED; ACR ID YOUR ON THE PARALLEL I REPLIED 'NO SIR; ACR ID JUST TURNED ONTO CARGO.' THERE WERE NO OTHER TRANSMISSIONS BTWN US AND THE GND CTLR. FACTORS: TWR INSTRUCTIONS WERE CHANGED (TAKE THE HIGHSPD INSTEAD OF ROLL TO THE END) AND GND INSTRUCTION WERE CLR. HE APPARENTLY WAS NOT AWARE OF THE CHANGE. TWR INSTRUCTIONS WERE INCOMPLETE AFTER CHANGING WHERE WE WERE TO EXIT THE RWY (I.E. HOLD SHORT 9R; STAY WITH ME?' WHEN WE WERE ON SHORT FINAL TO LAND ON 4R. TWR TOLD AN ACFT TO 'SHOW TO APCH SPD; YOU HAVE 80 KTS OF OVERTAKE.' IT WAS THAT ACFT THAT CAUSED THE TWR TO CHANGE OUR EXITING TAXIWAY TO THE HIGHSPD INSTEAD OF ROLLING TO THE END. CTLR'S AT ORD HANDLE A LARGE VOLUME OF TFC; MAYBE TOO LARGE. AT VARIOUS TIMES (TWR; APCH AND GND); ARE SO BUSY TALKING THAT IT IS IMPOSSIBLE TO ACKNOWLEDGE CLRNC. OCCASIONALLY THE CTLR IS TALKING CONTINUOUSLY TO SEVERAL ACFTS AT ONCE; MAKING IT VERY DIFFICULT TO COPY AND VERIFY YOUR CLRNC.