There's no way for the Soviet Union to have superior scientists? Or rather just be ahead research wise? Granted, yeah, US will need to be unable to research for some reason.
Also, Stalin trusted his scientists less than his spies, and as a result, any time developments in Soviet research contradicted the spy reports coming in from Oak Ridge and Los Alamos, Stalin would order his scientists to ditch their data/methodology/designs and use the American ones instead. That sort of limited how fast the Soviet team could work.
Indeed, there's a good argument that they could have got the bomb quicker if they hadn't had to copy Fuchs' Fat Man.