MTU size

Don't touch the MTU. Comcast uses the standard ethernet value of 1500. Changing it would only be covering up for something else that is wrong with the network.

Not true

Here is how to change MTU (for better performance in most cases)

First you need a site to ping. a good one is startpage.com

open cmd (admin)

at the prompt type:

ping your site, folloed by "f" and then the letter "L" and then the 1500 mtu that EG says is standard

Pinging startpage.com [64.71.134.108] with 1500 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 64.71.134.108:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

As you can see 100% loss at 1500 MTU this is because it has to be broken up.

Now do the same command but change the 1500 to something smaller, I did mine at 50pts

1450, 1400 1350 etc.. until you see something similar to the following:

Pinging startpage.com [64.71.134.108] with 1300 bytes of data:
Reply from 64.71.134.108: bytes=1300 time=66ms TTL=47
Reply from 64.71.134.108: bytes=1300 time=65ms TTL=47
Reply from 64.71.134.108: bytes=1300 time=66ms TTL=47
Reply from 64.71.134.108: bytes=1300 time=67ms TTL=47

Ping statistics for 64.71.134.108:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 65ms, Maximum = 67ms, Average = 66ms

bingo 0% loss

Now work your way up again until it bottoms out. The number before the 100% loss is your ideal MTU

Now to set it, do the following

Type the following:

netsh interface ipv4 show subinterfaces
   MTU  MediaSenseState   Bytes In  Bytes Out  Interface
------  ---------------  ---------  ---------  -------------
4294967295                1          0          0  Loopback Pseudo-Interface 1
  1500                    19705298    1920942  Wireless Network Connection
  1500                    5          0      14432  Wireless Network Connection 2
  1500                5          0          0  Bluetooth Network Connection
  1500                5          0          0  Local Area Connection

Now just tpe:

set subinterface “Wireless Network Connection” mtu=1350 store=persistent

replace the "" section with the network you want to change.

reboot and test.

I had a problem with mine, and could not get above 39 mbps - 75mbps
Changing my MTU, it is now at 149mbps - 175mbps (I pay for 200mbps so its not going to get that high on wireless, but this is much nicer than 39mbps)

We are not all automotons. applying an industry standard is not for everyone. places, equipment, computers etc.. all play a part. 
There are other things you can change in your speed, but this is the best.