Which type of course is the fastest for a 10 km TT? A completely flat course or a course with 5 km uphill and 5 km downhill?
It is pretty obvious. Just imagine the hill rose a bit, say to 5%. And even more, to 10%. Going across a mountain is never fast. Mountainous race stages are slow. The total meters needed to be climbed over the stage count, not just the different between the start and the finish.
Do remember that the average speed is not averaged over the distance but over time. So if you spent more time in the slower section, it counts more. And you do, because you are, well, slower...
There is an old riddle. You have one hour to go to a certain place 10 km away and back. So you plan to go 20 km/h. But it was uphill on the way there and you only went half the planned speed. How fast do you have to go on the way back to get back in time?
I also notice you ask about a light rider. The more there is meters to be climbed per every kilometer of the course, the better for a light rider although the time trial discipline has its specifics and specialist riders.