Harry Turtledove also wrote a series of alternate history novels (called the world war series) based on a similar idea. In it, the aliens’ civilization progress

Harry Turtledove also wrote a series of alternate history novels (called the world war series) based on a similar idea. In it, the aliens’ civilization progresses very slowly. A few centuries of human progress would take them millennia. They send a probe to earth in the middle ages, mark earth for an easy conquest, and show up during world war 2 where they find an almost evenly matched adversary. It’s pretty contrived, but an easy and amusing read.