Population, or citizen, growth is very strange. It wasn't noticed until 0.70 because that's when a value in the xml was changed. This caused the population growth to take many more turns. I did a lot of testing that was reported on Discord. That value will be changed to be much closer to the original value in the next update.
Other players were reporting what you have seen, so I did some testing of that also. It showed very clearly that the population increase took more and more turns as the population increased. It was always there, but it wasn't very obvious until the change that was made in 0.70.
In version 0.65, when the population of a planet increased from 5 to 8, the turns to get a new citizen increased from 10 to 17. In version 0.70, the turns needed to get a new citizen increased from 59 to 291.
I'm not sure what the 5.55 value does. It doesn't seem to have a lot to do with how long it takes to get the next citizen.