B757-200 CREW GOT ON THE BACK SIDE OF THE PWR CURVE; AND WAS REQUIRED TO DSND TO RECOVER AIRSPD.

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DUE TO INTERMITTENT TURB AT FL370; WE DECIDED TO CLB TO FL410. THE FMC INDICATED IT COULD BE DONE AND ATC AND PIREPS INDICATED THE RIDE WAS SMOOTH. THE CLB TOOK A BIT OF TIME DURING THE LAST 1500 FT TO FL410 DUE TO TURB. THE ACFT WAS AT TURB PENETRATION SPD. WE REMARKED THAT WE MIGHT WANT TO LET ATC KNOW ABOUT THE SLOW CLB. ALSO; THE ACFT HAD THE L ENG ANTI-ICE LOCKED OPEN DUE TO MEL RESTR. AFTER LEVELOFF AT FL410; THE ACFT ACCELERATED SLOWLY TO CRUISE SPD. ACFT WT WAS ABOUT 184000 LBS. SEVERAL MINS LATER; AFTER LEVELOFF; I TOLD THE CAPT I WAS GOING TO TAKE CARE OF SOME OTHER THINGS; AND TO WATCH THE ACFT. A FEW MINS LATER; I HEARD HIM SAY; 'HOW DID WE GET SO SLOW?' THE ACFT HAD SLOWED SOME 35 KTS FOR NO REASON. ALL PROPER MODES WERE ENGAGED. WE ASKED FOR AND RECEIVED A DSCNT TO FL370 TO RECOVER THE LOST AIRSPD. WE USED A MAX THRUST SETTING UNTIL DSCNT CLRNC WAS RECEIVED. THERE WAS NO STALL WARNING GENERATED; NOR WAS THE ACFT INDICATING ANY BUFFET. NO HARSH MANEUVERS WERE MADE TO REGAIN AIRSPD. WE RECHKED THE ACFT WT AND BAL AND THE NUMBERS WORKED OUT. THE FMC WT AGREED WITH A MANUALLY CALCULATED WT. THE FMC STILL INDICATED A MAX ALT OF 41500 FT. THE ONLY THING I CAN FIGURE IS THAT 1 OF 2 THINGS COULD HAVE HAPPENED: SOME KIND OF MOUNTAIN WAVE; OR THERE WAS NOT ENOUGH THRUST AVAILABLE FROM THE L ENG DUE TO THE ANTI-ICE BEING ON. THERE WAS NO ALT RESTR LISTED OR WARNING THAT YOU MIGHT NOT BE ABLE TO FLY AT ALT FOR A GIVEN WT. THEIR ONLY PWR CORRECTION APPLIED TO CLB SETTINGS.