Both have 4 cores, one uses the ARM instruction set and the other uses the x86 instruction set. There is little to deduce as far as performance is concerned simply from the instruction set used. Note that there's really no such thing AFAIK as an x86 quad-core, they're all 64-bit processors, i.e. x86-64 (a.k.a x64 a.k.a amd64).
Why your tablet sucks at multitasking does not derive from the instruction set it uses. At the rate at which mobile SoCs currently evolve, Tegra 3 is simply a little antiquated by today's standards.
I'm confused as to what you mean by "there's really no such thing as an x86 quad-core, they'll all 64-bit processors)"? x86-64 is a specification extension to the original x86 specification. It was designed (very cleverly) to be backwards compatible with the original x86 specification. Architecturally it is x86 at the end of the day; just with ISA additions. And what's more If AMD hadn't done it, we wouldn't be running x86 machines today and instead would be running IA-64 machines, emulating the original x86 ISA, and doing a poor and slow job at it to boot.
As a corollary, I wonder how well Intel would be doing today if they had stuck with IA-64... I have a feeling they would be not positioned as the global leader they are today if it hadn't been for AMD's extensions.