Slow turn-on time of e.g. ADXL103/203?

The ADXL103/203 (1 or 2 axis accelerometer) block diagram is shown below.

The turn-on time is listed as 20ms. A footnote says “Turn-on time is approximately 160 × CX + 4 ms, where CX is in μF” – i.e. part of that 20 milliseconds is due to the time needed to charge that external capacitor CX.

However this doesn't explain the 4 milliseconds. Where does this come from? The AC Amp, Demod IC, Output Amp, all should turn on 1000x faster than that right? Why the 4 milliseconds? I need to understand the ultimate turn-on speed limitations.

Secondly, what's the fastest turn-on time capacitive MEMS accelerometer out there?

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