I'm hoping someone has some insight into this problem, as it's got me stumped.
Comcast recently doubled my Blast! speeds from 30/6 (I'm guessing at the old speed tier) to 50+/10+ Mbps. After rebooting my less-than-a-month-old Motorola SB6121 modem and my Trendnet TEW-633GR router, I ran a speed test at speedtest.comcast.net from my laptop (connected wirelessly) and got around 53-54 Mbps down, and 10.3 Mbps up. As you might imagine, I was pretty psyched.
Yesterday I thought I'd test my wired PCs, one an old Pentium 4 running Win XP Pro with a D-Link gigabit Ethernet adapter, the other a modern Core i5 with 8 GB of RAM on an ASRock P67 Extreme6 motherboard, which includes two gigabit Ethernet ports (Realtek 8111E chip, according to ASRock's site). The XP Pro PC was pathetically slow (15/5 Mbps), but after running the latest version of the TCP Optimizer (Optimal settings, ISP speed set to 50 Mbps), and then rebooting it and the HP ProCurve switch, its speeds jumped to 38/9 Mbps, and those speeds are the same today. And given the outdated D-Link NIC in it, that's all good.
The Windows 7 Home Premium PC, which the Toshiba laptop also runs, and also runs the same Core i5 CPU, is a different story. Despite comparing the various TCP parameters to the laptop's via the netsh command, and despite running the latest TCP Optimizer on it, it never gets above 25 Mbps downstream. It consistently gets 10+ Mbps up. I've also tried updating the NICs' firmware, trying each NIC separately, as well as taking the switch out of the equation and connecting it directly to the router, but the results are the same.
Additionally, I tested an older duo core (Core Duo?) HP laptop, and it, like the newer Toshiba, gets the advertised 50/10 Mbps. I should note that both laptops and the router are connected via wireless "N," but that's rather beside the point, as my entire LAN is gigE and running jumbo packets of 9K.
To rule out the cable, I copied a 2 GB file from a Netgear ReadyNAS Ultra 4 to the PC at over 100 MBps (not Mbps).
The TCP parameters on the PC match those of the laptop, both in the netsh command and in the registry, with the exception of one registry parameter available to the PC's NICs that is not available on the laptop.
At this time, I can't connect directly to the modem, and I doubt that would make a difference.
The last thing I checked was the router's configuration, specifically for QOS settings. It's all pretty neutral. It is running the latest firmware.
So pretty clearly this is not a Comcast problem, not a modem problem, and I'm guessing not a router problem. It seems to me it's either the PC's NIC(s) or some network configuration issue I'm overlooking, but as noted, on the LAN I get blazing speed.
If anyone has any ideas, I'm all ears.
Thanks in advance.