reduction in grow rates only will be not enough i think.
If you reduce only the grow rate so you will have all city´s large or huge after 30 years and not 20 as now...its no really way to fix it. The only way i think is to allow only few cities to going large or huge at best you allow only lv 3 palace for most cities, for some desert oase´s only lv 2 or 1 and for few like rome,athen,karthago,antiochia etc. lv 5.
This is a good idea