The MTU (Maximum Transmission Unit) describes the maximum packet size of a protocol of the network layer (layer 3) of the OSI model.
On the device that generates the PPPoE, we recommend an MTU of 1492 (Swisscom often recommends 1460). If packets are unnecessarily fragmented, this can have a significant impact on performance. Typically in the upstream, as this is where the MTU plays a role.