You can still get up to 25-30Mbps with a docsis 2.0 modem like that one, and it would not affect your latency unless it is malfunctioning. I only went from that modem to a SB6121 when I got a 50Mbps connection, the SB5101 worked fine up to 25Mpbs.
That said, it is an older modem and could be breaking down, but an equally likely issue is with your ISP. What kind of latency and jitter did you get from pingtest.net?