A planet with 15 production will take 2 turns to build a factory. Great. All else being equal, it'll take 4 turns for 2 factories. Cool.
A planet with 29 production will take 2 turns to build a factory. Great. All else being equal, it'll take 4 turns for 2 factories. What??
This means if you want to actually make good use of your extra production, it absolutely necessitates micro management of almost every planet, almost every turn. I want that extra 14 production to do something, so I allocate it to shipyards/research/wealth... manually, via pixel-click hell.
Please, put that extra production to use in building the next thing. Same goes for research.