> Is that even theoretically possible, assuming we don't do the obvious thing and make normal CPUs slower to force it to be true?
No. With all the space they waste being reconfigurable, the max frequency they can support at the logic element level is much lower just due to how much distance the signal has to travel without attenuating too much.
Less about physical space, more about propagation delays. The configurability of an FPGA doesn't come out of nowhere; it's a bunch of multiplexers and pass transistors which route signals around and control how they're processed by CLBs, and those elements all add delay, limiting Fmax.
No. With all the space they waste being reconfigurable, the max frequency they can support at the logic element level is much lower just due to how much distance the signal has to travel without attenuating too much.