Why does the very first transformation of my image sometimes takes a little longer?

Cloudimage Filerobot

This is a normal behaviour and you should not be worried about it.

As per Image Optimisation architecture, your image is first downloaded from your origin storage over HTTP(s), cached, transformed and spread out to all CDN Points of Presence (PoPs). Once cached in the CDN, the image is delivered at light speed for every subsequent request.

Whether you use Cloudimage or Filerobot, the process will be the same.