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.
Poster salientis has received an official response from Motorola regarding the issue, acknowledging its existence:
"Engineering is still investigating, but has identified a possible issue between Apple computers and certain networking configurations. Due to the fact that multiple devices are involved, the root cause is not yet known. We are continuing to investigate and will take the appropriate action once the cause and correction are known."
So far, the best workaround for this issue appears to be use of a modem from another manufacturer, or an older Motorola modem (outside the 4000 and 5000 series')
If you use a Motorola SurfBoard modem and are experiencing slow throughput, please let us know.Resources