Taken from Balance Discord:

:regional_indicator_q: What Skillspeed do I want?
:regional_indicator_a: :blobwizard: Going with this as my source:
-1523 for 2.38 GCD - Base comfy
-1624 for 2.37 GCD - Upheaval is comfy
-1726 for 2.36 GCD - "Greeding Mechanics" levels of comfy
-1827 for 2.35 GCD - "I love breathing room" levels of comfy
More about SkS:

Anything Above / Below those tiers are fine, but I am not going to bother with mapping special rotations. Use the link in the post to find breaking points for your preferred GCD.
Use the "2.5s" column to see what your GCD becomes with X amount of Skill Speed.