Fragmentation that occurs on intermediate routers is clearly detrimental to the CPU resources on the router responsible for fragmenting an existing datagram. This is even more true today than it was in 1987, as routers have implemented more functionality in firmware and specialized hardware. Routers now optimize for the normal case. They simply transfer a packet from one interface to another in firmware while decrementing the ttl of the packet's header. Therefore, routers now exhibit an even larger relative penalty if they have to do any significant processing on the packet, especially if fragmentation routines are implemented in software rather than firmware.
It would obviously be a mistake, however, to regard all fragmentation as harmful simply because intermediate fragmentation is a burden on routers. Endpoint fragmentation with pathMTU discovery imposes no additional processing burden on the intermediate routers. Each fragment is formed at the endpoint to be no larger than the pathMTU. Since all fragments are forwarded as-is with no additional processing required.