A C310 PLT; ON A TRIP FROM PAE TO RNO; ENCOUNTERED FORECAST ENRTE WHICH CAUSED HIM TO MAKE SPD ADJUSTMENTS AS HE WAS UNABLE TO MAINTAIN ASSIGNED ALT.

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IN THE PLANNING OF AN IFR FLT FROM PAINE FIELD; EVERETT TO RENO INTL IN A C310 ALONG WITH 2 PAX; I DECIDED TO GO ON A DAY THAT HAD A LULL IN THE WX ALONG THE RTE. BOTH PAE AND RNO HAD VFR FORECAST FOR THE PERIOD. THE FLT WENT WELL AS WE HAD A SMOOTH RIDE BTWN LAYERS OF CLOUDS; BUT EXPERIENCED PROBS ABOUT 10 MINS FROM OUR DEST (RNO). AT THAT POINT; WE WERE FLYING AN ASSIGNED ALT OF 14000 FT; WHEN; OVER A PERIOD OF APPROX 5 MINS; WE EXPERIENCED A SIGNIFICANT BUILDUP OF RIME ICE. OVER THAT PERIOD OF TIME I NOTICED THAT OUR ACFT PERFORMANCE DEGRADED TO THE POINT WHERE I COULD NO LONGER MAINTAIN OUR ASSIGNED ALT. I ADJUSTED THE ACFT'S AIRSPD AND FOUND A POINT THAT MINIMIZED THE RATE OF DSCNT WITHOUT COMPROMISING MY ABILITY TO CTL THE ACFT AND ADVISED ATC OF THE SIT. THEY ADVISED ME THAT A LOWER ASSIGNED ALT OF 10700 FT WOULD BE AVAILABLE IN APPROX 9 MI (3-4 MINS). IN THE MEANTIME I WAS ABLE TO STOP THE RATE OF DSCNT AND LEVEL OUT AT APPROX 11600 FT. I HELD THIS ALT UNTIL A CLRNC WAS RECEIVED TO DSND AND MAINTAIN 10700 FT. I WAS THEN ABLE TO MAINTAIN ALL SUBSEQUENT ALT ASSIGNMENTS; SUCCESSFULLY COMPLETING THE ILS APCH AND LNDG WITHOUT FURTHER INCIDENT. IN REVIEWING MY PREFLT PLANNING AND WX BRIEFING PRINTOUT FOR THIS FLT; I DISCOVERED THAT I HAD OVERLOOKED AN AIRMET ON ICING FOR THE RNO AREA. TO AVOID THIS KIND OF PROB IN THE FUTURE; I PLAN TO MORE CAREFULLY SCRUTINIZE PREFLT PLANNING AND WX BRIEFING TO DO ALL I CAN TO AVOID ICING CONDITIONS. A GENERAL RULE THAT I PLAN TO USE IN THE FUTURE IS NOT TO FLY IN CLOUDS OF PRECIPITATION WHEN THE FREEZING LEVEL IS ON THE GND ALONG THE PLANNED RTE. IT IS AMAZING HOW THE PERFORMANCE OF A HIGH PERFORMANCE ACFT CAN BE SERIOUSLY DEGRADED BY THE BUILDUP OF ICE ON THE AIRFRAME.