The problem with audio processing has always been latency. Are there functional programming languages out there which can guarantee bounds on latency?
Galois has done work in Haskell for developing hard real-time applications. Here’s a dataflow language they built for example: https://leepike.github.io/pub_pages/rv2010.html
The best marriage of functional concepts and realtime I've seen is Supercollider (https://supercollider.github.io/) which basically has a smalltalk-like lang control a realtime backend. I'd love to see something like this for Haskell + Faust!
It compiles down to deterministic C++. Each frame of audio takes exactly the same computation path; ditto for each sample within each frame. (Although some parameters might be updated once per frame, and others updated once per sample.)
You could inspect the output C++ and count up the number of cycles for each frame of audio, if you wanted.
faust programs basically define a signal flow that gets compiled down to various languages, there won't be any latency besides the one that may be part of your own algorithm.
Math operations on a DSP/CPU require cycle time. Cycle time can vary, but even a simple comparison can be 1-2 cycles, which could be nanoseconds of latency.