Check constraints and function volatility categories

Hello,

I'm discovering that I need to write quite a few functions for use strictly
w/ check constraints and I'm wondering if declaring the volatility category
for said functions will affect their behavior when invoked by PostgreSQL's
check constraint mechanism.

Essentially what I'm trying to figure out is if volatility categories
increase or decrease the latency of executing check constraints. I've done
some micro benchmarks but I have no experience benchmarking anything in
PostgreSQL to trust that I'm measuring the right thing. So I'm asking the
experts.

I'm running PostgreSQL 9.5 on Ubuntu Linux 15.10. I don't know if this
matters but this is my workstation which is a pretty zippy AlienWare X51 w/
16GB RAM on a Core i7-4770 processor.

Thanks,

Dane