CAPT OF AN MLG WAS UNABLE TO HOLD ASSIGNED HOLDING ALT DUE TO EXCEEDING THE ACFT'S OPERATING LIMITATIONS. THEREFORE; SINCE ARTCC CTLR COULD NOT ASSIGN LOWER ALT DUE TO TFC; EMER WAS DECLARED AND LOWER ALT PROVIDED.

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ATC ADVISED US TO HOLD S OF BOWLING GREEN; 175 DEG RADIAL; 20 NM LEGS. ACFT WAS FLYING AT FL350. CAPT KNEW ACFT WAS A LITTLE HVY FOR HOLDING AT FL350 BY CHKING HOLDING AIRSPD/ALT CHART. CAPT ELECTED TO STAY AT FL350 IN ORDER TO CONSERVE FUEL. INITIALLY ACFT RESPONDED WELL IN TURNS IN HOLDING AT FL350. ON SECOND TURN IN HOLDING; CAPT NOTED THAT ACFT WAS UNABLE TO HOLD AIRSPD AT MAX THRUST. CAPT ADVISED THE COPLT TO INFORM ATC THAT THE FLT NEEDED A LOWER ALT. ATC INFORMED FLT THAT LOWER WAS UNAVAILABLE. SEVERAL RADIO CALLS FOLLOWED FROM CREW INFORMING ATC THAT LOWER WAS REQUIRED BECAUSE THE ACFT WAS UNABLE TO MAINTAIN SPD. ATC DID OFFER TO ALLOW ACFT TO DRIVE STRAIGHT INSTEAD OF TURNING IN A HOLDING PATTERN; BUT BY THAT TIME THE ACFT WAS LOSING AIRSPD AND WAS UNABLE TO REGAIN IT. AFTER SEVERAL MORE CALLS THE CREW WAS INFORMED THAT LOWER WAS ONLY POSSIBLE IF THE FLT DECLARED AN EMER. BEFORE AIRSPD REACHED AN UNSAFE PERIMETER; THE CAPT DECLARED AN EMER AND DSNDED TO FL340. CREW REQUESTED FL310 AND WAS SUBSEQUENTLY CLRED TO FL310. WHEN LEAVING FL350; ATC ADVISED OF TFC WHICH WAS SEEN BY CREW. NO TA OR RA ADVISORIES WERE EXPERIENCED DURING THE EVENT. ONCE AT FL310; ACFT WAS ABLE TO HOLD AT PROPER SPDS AND PWR SETTINGS.