PPPoE adds a small amount of overhead to each packet sent over the Internet, which slightly reduces the optimum packet size. That wouldn't be bad if it weren't for the fact that some PPPoE implementations make it hard for the operating system to figure this out, resulting in packet fragmentation when packets are bigger than the optimum size. Even worse, packet fragmentation can make an Internet connection unreliable, unstable or both.

You can easily check your MTU at the dslreports.com Tweaks page, and adjust it as needed. (If you want to set it manually, try settings of 1492 and/or 1454.)