MTU defines the largest packet of data which can be handled without fragmentation (i.e. breaking the packet into two or more smaller packets). If an incoming packet is (say) 1460 bytes and the router MTU is 1450, then the incoming packet will be fragmented into two packets, the larger one being 1450 bytes. Now, if the PC MTU is 1440 then the already fragmented 1450-byte portion of the incoming packet will be fragmented again. This fragmentation slows down data transmission.
The largest packet which is used is 1500 bytes, so if the router MTU is set to 1500 it will never fragment packets travelling in either direction, and this is the best technical option.