It is really hard to compare real-world performance for CPU's with such drastically different architectures without benchmarks (and even with benchmarks it is difficult.)
One way to measure theoretical CPU performance is in DMIPS though (basically how many million instructions per second can the processor perform after considering differences in instruction sets by a generalized benchmark called Dhrystone.) An instruction set is the set of all instructions that the CPU's machine language provides for.
So for the A57 the recorded statistic is 4.1-4.5 DMIPS/MHZ. Let's just take it to be 4.3 DMIPS/MHZ. Mutiply that by a clock speed of 1020 MHZ, and we get 4182 DMIPS/core.
Scouring the web it looks like Expresso is 2877.32 DMIPS/core
The ratio of performance is then 4 cores*(4182) DMIPS/core/ 3 cores*(2877.32)DMIPS/core = 1.93 times more instructions/sec for Switch's cpu than Expresso.
So more or less twice as many instructions per second, on a basic comparison.
The Xbox 360's CPU provides 5638.90 DMIPS @ 3.2 MHZ for all cores
Which gives a ratio of (4*4182)DMIPS/5638.90 DMIPS or about 3 times the Xenon.
Not even going to bother comparing to the CELL because the architecture is so odd.
Jaguar has about 3.6 DMIPS/MHZ , so 1,750 MHZ * 3.6 DMIPS *8 cores = 50,400 DMIPS (for XBOX ONE)
So the Switch's CPU is about a 33% the Jaguar @1,750 MHZ (assuming both use all of their cores.)
Jaguar in the PS4/XBO ~ 3* A57 in Switch; A57 in Switch ~ 2*Expresso in Wii U ~ 3*Xenon in Xbox 360 (theoretically; assuming all cores can be used to the max.)
Performance is of course different, because we know Microsoft and probably Nintendo don't use all of their cores at max.
Also note that developers have commented that the Xenon has better real-world performance than the Expresso, but that could just be a matter of not bothering with optimization for the Wii U's advantages in ports.
