precedence of setInterval, eval and print execution

Does the server connection stay open, or is that opened for each request as well?

Definitely pushing data over the TCPIP to the console as you are is going to have some significant overhead - the data has to come in over serial from the ESP8266, get parsed, pushed back into the IO buffer for the console, handled for the console, executed, and then the result printed. However that shouldn't be anywhere near 100ms - and in fact a lot of that work should be spread out so wouldn't delay the execution of setInterval.

If I created a mini-repl within the TCP connection to do the 'swch.getValue()' call and return the result upon receipt of a magic word, without the LoopBack pipes, the duration of the disturbation might be significantly shorter.

That'd definitely help. I wonder if there isn't something else going on though... Could you maybe send me the full code in such a way that I could try it out here where I do have access to the device?

I'm wondering if there is a way to have a function interrupt currently running console code without changing the Espruino src code.

For analogRead one option is to use Waveforms: http://www.espruino.com/Waveform

The analog read will then be performed in an interrupt and saved to a buffer. I haven't tried this but you may be able to double-buffer with just a single-element 16 bit buffer, which would allow you to get accurate timings on the readings.

Interrupting some JS to execute other JS isn't really something that's possible right now, but it is possible to do it with Assembly, inline C, or compiled JS and setWatch(...irq:true). I really think that for this, that would be overkill though!