Motorola SB5101u cable modem

[Fbsd8]

Having very slow speeds on my LAN. Looking for cause.
I have a D-link DIR-655 10/100/1000 router behind the cable modem.
The specs on this Motorola SB5101u cable modem states it has 10/100 
mbps, but then it also says "internet download speeds up to 38 mbps and 
upload speeds up 30 mbps.
I am paying my ISP for 4 gbps down and 2 gbps up service.
Is this Motorola SB5101u cable modem a bottleneck causing my slow lan 
speeds?