fps drop when arty appears

hello guys i got a problem with the arty,makes me drop so many fps i got like 50-60 fps and when arty appears gets like 30-40,my spec

proccesor is an amd phenom x4 955 3,20ghz

4gb ram

gt 640 2gb

i use low setting or med and i got the same problemD:!

anyone can help me??:c

cheers!