This reminds me of various Windows applications back in the day (Windows 3.1 and 95) that claimed to fine tune your connection and one of the tricks they used was changing the MTU setting, as far as I can recall. Could anyone share how that worked?
If your computer sends a larger MTU than the next device upstream can handle, the packets will be fragmented leading to increased CPU usage, increased work by the driver, higher I/O on the network interface, higher CPU load on your router or modem, etc depending on where the bottleneck is. For example if you connect over Ethernet to a DSL modem, or to a router that has a DSL uplink, all your packets will be fragmented. This is because DSL uses 8 bytes per packet for PPPoE authentication. So if you send a 1500 byte packet to the modem, it will get broken up by the modem into 2 packets: one is 1492+8 bytes, and the other is 8+8 bytes.
But your PC is still sending more packets.. the modem is struggling to fragment them all and send them upstream.. its memory buffer is filling up.. your computer is retrying packets that it never got a response on..
By lowering your computer MTU to 1492 to start with, you avoid the extra work by the modem, which can have considerable speed increase.
