Ribbed pipes reduce the flow to the minimum diameter of the ribs at the narrowest part of the ribbed section plus a significant amount more since the turbulence between the ribbed sections reduces the effective laminar flow region substantially. A ribbed coolant tube can have as little as 40% of the flow cross section flow of a smooth tube.
In other words, it can be like replacing a 3” diameter radiator hose with on that’s only 1.25” in diameter.