The speed of your cable modem is determined by the DOCSIS standard it supports and the number of channels that it offers, although your ISP also has to support these standards on the other end. Buying a DOCSIS 3.1 cable modem won't give you any better performance if your cable provider only supports DOCSIS 3.0, although it could still be a good investment for the future. Further, even though 32-channel DOCSIS 3.0 modems offer theoretical speeds of up to 1 Gbps, most cable providers top out at 600 Mbps over DOCSIS 3.0, so if your ISP is offering multi-gigabit plans, you'll almost certainly need a DOCSIS 3.1 modem to take advantage of those speeds.