I play on faster and I don't think it's your imagination. Playing at day time in June at Shea Stadium yesterday in a play now game and the game was buttery smooth and amazing.
Playing in playoff mode ( which causes crowds to be packed, plus have more animations go on, at night time games ) at different ballparks, the game still seems to typically play at 60fps or so but it has occasional drops but still doesn't seem as smooth although it's playable.
They made definite improvements over last year, where I tried March to October and played opening day against the Nationals in Nationals Park and the game's frame rate was horrible last year, but it still doesn't seem to always be 60FPS smooth at all times in some ball parks at night time.