Off the top of my head, when the PowerMacs using the PPC 601 CPU were introduced, this situation occurred.But no machine that I can think of off the top of my head was replaced with something slower. Of course a low end system was going to be slower than a high end system from one or two years ago.
At the time, Apple did not have a native running system for the PPC architecture so they emulated the Mortorolla 68 series until the system could be ported over. So the early 601 based systems were slower on many tasks compared to older 68040 based systems.