There appears to be an issue affecting some Motorola SurfBoard cable modems that results in lower throughput speeds when the devices are connected to Macs than when the devices are connected to Windows-based PCs.

Several posters to an Apple Discussions thread note that Motorola Surfboard 4200, 5100 and possibly other models deliver significantly lower throughput on Macs when compared to PCs -- generally with Macs achieving about one-third of the bandwidth available to PCs.

Continues @ MacFixIt