In comparison with Halo 3 and ODST, there's the sense that for the vast majority of the duration, Reach is the smoother game. While frame-rates in optimal conditions are identical, the new game's implementation of motion blur - both camera and per-object based - is markedly superior to what we saw in the old engine. Put simply, the game just looks that much more fluid in motion.
However, on the odd occasion where frame-rate drops, there's the definite sense that performance lags behind both Halo 3 and ODST at their worst. The question is why? Fill-rate is the likely culprit, as opposed to bandwidth or geometry concerns.