Apple's A-series processors have outperformed the Wii-U for some time. The 3rd Generation iPad in 2012 matched the PS3 on performance. Because the Wii-U is on par with the X360 and PS3 the mobile devices have been ahead for some time now.
It gets worse for Nintendo when you consider the slow and clunky processor in the Wii-U. The Wii-U has in it a triple core G3 processor (nearly unchanged since 1998) clocked at 1.2ghz per core running 32-bit code. The A7 in the iPhone 5S is dual 1.4ghz 64-bit cores. In single threaded applications (games) this outperforms the ancient PowerPC by a long way. In multithreaded applications (operating system services) it matches if not exceeds performance.
There's also the question of heat and clock speed. The Wii-U's processor requires a cooling system (heatsink and fans) while the A-series can run without one by underclocking. If you attached a cooling system to the A7 you could possibly (in theory) push it to nearly 2ghz. Additionally the A-series have a 64-bit address bus (meaning in theory it can support more than 4gb RAM + ROM) while the G3 is only 32-bit (Maximum 4GB RAM + ROM). It's one reason why the Wii-U had no more than 2gb RAM - with the other half of the address bus dedicated to ROM and I/O they couldn't add any more memory to it.
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