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Speed drops with use of any router/firewall

My father in Kansas City has Time Warner for internet/TV. From what I understand he recently had his account upgraded to 15MBps, however he could not get past 10MBps. Well turns out the router I had given him (old Cisco 1721) was limited to 10MBps routing anyways. This weekend we came for a visit, and I brought with me a Sonicwall TZ100. Plugged that in hoping solve this issue, only find that his speeds have actually gotten worse.

Here is the annoying part. Directly connecting laptop to modem, get 15+Mbps. Plug in firewall, get 1-2Mbps. First thought was maybe cable was bad. Yet using that with laptop to modem, I get 15+Mbps. Second thought would be, is this issue with Sonicwall? No, this tests out fine.

For whatever reason, a router in between cable modem and devices drops the speed significantly. I should add, I have Comcast 30MBps at home, and with these devices I get around 30 with Sonicewall, 10 with 1721. So if cables are good, routers are good....why or how could such a drop in performance be happening?

What model modem?

Cisco 2100
Note:
Name: CISCO DPC2100R2

Hardware Version: 2.1
Software Version: dpc2100rx-v202r1256-110531Uas-TWC
Receive Power Level: -6.0 dBmV
Signal to Noise Ratio: 37.7 dBmV
Transmit Power Level: 50.8 dBmV
Cable Modem Status: Operational

Edit: Also tested, put 1721 router between two laptops, and did file transfer. Sure enough got about 10Mbps.

Is your Firewall / Router for any reason negotiating a 10Mbps Half Duplex connection to the Cable modem?

Do you have any kind of qos enabled?