It occurred to me that, in the conventional opamp all-pass 90 degree phase shifter circuit,
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a fast signal passes through all the opamp stages, so all the amps have to be fast. In this one,
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the fast signals just rip through the big caps in the slow stages. So in a high-order shifter we could use slow, cheap, low-noise amps in the first few stages and just use fast opamps towards the end. That will make less HF distortion too.
The down side is that the output impedance isn't low, like an opamp would be, so cascading stages gets a little more difficult, or more interesting depending on your attitude.