There's no real way to directly compare the Switch CPU with the last-gen consoles, because they use completely different architectures.
However if you're going to try, last-gen CPUs ranked like this PS3 > Xbox 360 > Wii U.
The Wii U's CPU was much weaker than the PS3 and 360, what made it competitive was having a newer architecture and having OoOE, but even then it was still weaker. The Wii U had a 3-core 1.24 GHZ processor, Xbox 360 has a 3-Core / 6 thread 3.2 GHz processor, and PS3 had the Cell which was a completely unique setup best explained as a PPE (aka 1-Core) / 8 SPE (aka 8 super threads) 3.2GHz processor (however, 1 SPE was disabled and another for OS).
The Switch is using a mobile CPU, 4-core ARM Cortex A57 at 1.02 GHz with OoOE. Core for core it should be be in a similar ballpark as the Wii U's Espresso CPU obviously with an additional core to use. It also has more current architecture which helps it even more.
If I were to guess I would say the rankings are like this.
PS3 > Xbox 360 = Switch > Wii U
There are more things we need to know like the Instruction set for all 4, channels, memory bus, and more, but based on what we do know that's my hypothesis.
