HIGH ASSIGNED CRUISE MACH AND PROGRAMMED LOW COST INDEX DESCENT SPEED RESULT IN FAILURE TO MAKE CROSSING RESTRICTION. A DISTRACTION DUE TO REPROGRAMMING A RWY CHANGE CONTRIBUTED TO LATE RECOGNITION OF THE PROBLEM.

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ON DSCNT INTO JFK VIA THE LENDY 5 ARR; THE CURRENT ATIS WAS RECEIVED AND AN APCH TO RWY 22L WAS BRIEFED AND SET UP. A CLRNC TO CROSS HARTY AT FL230 WAS GIVEN AND THE ACFT BEGAN A VNAV DSCNT TO CROSS THE FIX AT THE GIVEN ALT. PRIOR TO REACHING HARTY; A NEW ATIS WAS GIVEN AND A RWY CHANGE NECESSITATING A NEW APCH SET-UP AND BRIEFING WAS REQUIRED. AS THE PNF; I ACCOMPLISHED THE SET-UP. JUST AFTER THE APCH BRIEFING WAS COMPLETE; IT WAS OBSERVED THAT THE ACFT WAS NO LONGER IN VNAV PATH BUT HAD DEFAULTED TO VNAV SPD. IT WAS (ACFT) HIGH ON ITS PROFILE DSCNT TO CROSS HARTY AT FL230. AGGRESSIVE MEANS WERE USED TO MAKE THE XING RESTR. HOWEVER; THE ACFT WAS 400 FT HIGH WHEN XING THE ACTUAL FIX. ON REVIEW; IT WAS NOTED THAT THE CTLRS HAD WANTED AN INCREASED SPD ON THE ARR. THAT INCREASED SPD WAS PLACED INTO THE FMC ON THE CRUISE PAGE. WHEN THE ACTUAL DSCNT BEGAN; THE COMMANDED SPD DROPPED BY MORE THAN 60 KTS DUE TO THE FACT THAT A LOW COST INDEX WAS PLACED INTO THE FMC DURING PREFLT. THIS IS A COMPANY DIRECTED PROC TO SAVE FUEL DURING THIS ERA OF HIGH ENERGY PRICES WE LIVE IN. A SPD DECREASE OF 60-70 KTS FROM CRUISE TO DSCNT IS 99% OF THE TIME NOT APPRECIATED BY ANY CTR OR APCH CTLR. WE ARE THEN REQUIRED TO PULL OUT THE LOW COST INDEX DERIVED SPD AND MANUALLY INSERT THE PRACTICAL SPD REQUIRED NOT TO DISRUPT FLOW ON THE ARR. IF WORKLOAD IS INCREASED AND THIS CHANGE IS NOT MADE; A SHIFT FROM VNAV PATH TO VNAV SPD IS A REAL POSSIBILITY AND AN IDENTICAL RESULT WILL HAPPEN.