Perhaps G-Sync and a high frame rate is a simpler way to reduce latency, and then we can still keep the three frame pipeline. I don't think he mentioned G-Sync which was a surprise. JIT rendering also sounds good though.
That means, heat loss will be 10x greater.
However, from the perspective of brightness (rather than efficiency), you're right, 99.9% is only about 1.01x more efficient than 99%.
Presume the new efficiency was 99% and the old efficiency was 38%. Naively, that sounds like 2.6x better. However, it's far more constructive to look at it in terms of how close it comes to 100%. So really, the formula is (100 - 38) ÷ (100 - 99) which means it's not 2.6x more efficient, but 62x more efficient. To get the percentage improvement, we add a little more to the formula: (100 - 38) ÷ (100 - 99) × 100 - 100 = 6100% improvement.
With the original numbers we calculate (100 - 38) ÷ (100 - 60) × 100 - 100 = 55% improvement.
If you're looking for someone with STL experience who is expected to be up to speed and generating good, idiomatic code, then these types of questions can help weed out people who know C++, but don't have a lot of practical experience with the type of code your company wants. It's not the best interview question, but it's not terrible, either. Unless they expect you to actually list every single one. If you can spout off array, vector, and deque; that's a pretty good start.