What is the fastest time or shortest duration for reliable setInterval()

What is the shortest reliable duration?

A millisecond should be safe, but you should be able to get a bit less than that. Obviously if other code is executing that can delay execution, but if as you say your code has finished executing then you'll be fine.

If you need to get very fast, you might be able to use digitalPulse,etc. Another option is to set up a PWM output, connect it to an input, and then watch that input with setWatch(...., {irq:true}) and use compiled code.

I'm seeing limitations of the IDE keeping up around 80-100ms

Do you have an example of code that does this?