I tested it already. I have made now 2 stacks of re-enriched uranium cells and every time it takes just about 20 seconds to finish the process.
I am talking about a possible bug in the game, because the reactor heat at start is 3 and in about 20 seconds it produces 4 re-enriched uranium cells.
I gave me the depleted uranium cells via creative mode. I think it is not a bug related to creative mode because this just happens with my breeder. Other breeders still need a long time to finish one cycle.