That is not strictly true, the actual latency increase is half the frame rate. Because the input is not just the frame image but also the motion vectors (in which direction the pixel moved) for the current frame. Frame gen also knows a lot about the image, like which bits have transparent pixels (which move in multiple directions at once) and when the game is done with the frame yet still has to wait for the GPU (time which can be used for more work with little impact).

Frame gen is much more involved than the old “motion smoothing” of televisions, the so called “soap opera” mode, which did increase the latency much more and had no knowledge of how the source image was built, so processing was much more involved.

Stuff like DLSS5 is supposed to use the same inputs (source images and motion vectors), now that is magic to me.