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Oops, sorry, that's what I get when I don't read the actual reference. I had thought it the more commonly referenced router performance paper (attached).
A "WAN" circuit, more or less, just means a non-LAN circuit. It's expected to be full duplex.
For a 1921, the 15 Mbps means that Cisco recommends this router for up to 30 Mbps of aggregate bandwidth going to/from a "WAN". (A "WAN" may impose additional feature overhead beyond interior LAN routing due to typical [Internet] WAN features such as NAT or firewall, and/or different media, etc.)
The only usual difference a cable modem (or ADSL modem) imposes, is asymmetrical bandwidth. If you had a cable modem like 150/20, the "WAN" bandwidth equivalent would be 85 Mbps ((150+20)/2).
Back to the 1921, yes it's documented providing up to 2.77 Gbps, but that's best case. 1500 byte packets, and just packet forwarding. Also that's not 2.77 Gbps, duplex.
But for 64 byte packets, it's documented at 290 Kpps. As wire-rate for such sized Ethernet packets is 1,488 Kpps per gig, the 290 Kpps supports about 195 Mbps. That's still for just packet forwarding, but quite a drop from 2.77 Gbps. (It does show the impact of packet sizes.)
Once you start to add features, performance drops even more.
In your reference, Table 8 might be used to support "typical" usage. In that table, a 1921 is listed at 68 Mbps (aggregate) but that's much better than Figure 1's 15 Mbps (WAN - or 30 Mbps aggregate).
Cisco's recommendations are conservative, to insure the recommend router's performance doesn't disappoint, but real world often doesn't load links to 100% all the time, or use as many IOS features. Often you can "downsize" one two levels and not have any performance issues.
