It is well known that Class Freak isnt helping to negate the negative effects of Speed Demon mutation but Bethesda doesnt appear to know why.
I have just taken a Speed Demon and Eagle Eyes serums and this is a screen shot from directly afterwards.https://i.imgur.com/usJ9hJL.png
Mutations have a positive effect and a negative effect. When you first take a serum, the negative effect is suppressed and the negative effect isnt showing (hence why you dont see the -4 STR from Eagle Eyes).
But you can still see the +50% Hunger/Thirst showing when it should be suppressed.
Therefore the issue is the game is recognizing the +50% Hunger/Thirst as a POSITIVE effect, not a negative one.
That is why it's not being supressed when you first consume your serum and that is why Class Freak isnt affecting it.
External link →