Slow to start ruby

[resistorsoftware]

I just re-habbed an older iMac from 2015 with 4 cores to run MacOS v15. First thing I did was install Brew and a fancy terminal (Warp and Kitty). So with those I like the Starship prompt to show me which Ruby I am using, state of my Git and JS etc.

Second thing I did was install rbenv and then some Ruby code to go along with my developer life. Sadly, the addition of even a lowly file .ruby-version matching one of my installs takes almost a 1000ms to process!! On a similar install it is ~80ms

I am no expert at the way ZSH shell and rbenv play nice with shims and Ruby, but this strikes me as 10x slow and therefore makes my prompt in my terminals very sluggish too! All because answering the bell to the question Ruby?? is taking so long.

Can anyone inform me as to what mistake I have made in deploying Ruby with rbenv such that start-up is so slow? Is this an Apple MacOS thing perhaps? Any clues as to how to debug a slow ruby start-up?