Yes, slow/haste should not give less/more turns based on turns already taken. That is just counter-intuitive. It should only affect from the point in time the slowing/hasting spell is cast.
I don't get it, from the moment you cast a spell ALL the following turns are affected as they are "future" turns, the spell reorganises the order in which units will move or attack, it cannot change turns already taken as they are "past" turns (where are these already taken turns?)
as far as I can tell, slow and haste have a noticeable effect consistent with the initiative level of the units in play, even if it is sometimes limited
once I had five or six turns in a row, my sov had a 22 intitiative, the monster had only 6, lucky me, the wildland ice beast was pretty dangerous