The potential for exploiting rate variations to in-creasp the capacity of wireless systems by opportunistic scheduling has been extensively studied at packet level. In the present paper, we examine hon slower, mobility-induced rate variations impact performance at flow level accounting for the random number of flows sharing the transmission resource. We identify hvo limit regimes, termed /hid and qttmi-smimoy. where the rate variations occur on an infinitely fast and an infinitely slow time scale, respectively. Using stochzstic comparison techniques, we show that these limit redmes provide simple performance hounds that only depend on essily calculated load factors. Additionally, we prove that for B broad clnss of fading pm-cesses, performance varies monotically with the s p e d of the rate variations. These results are illustrsted through numerical erperiments. showing that the fluid and quasi-stationary bounds are remarkably tight in certain usual CBSCS.