Man Flex
Correct, longer in the descent with idle...rather than at cruise with 80-90% N1. You've surely noticed how a slower decent means TOD comes sooner...maybe 5 minutes sooner...saving 5 minutes of cruise power fuel.
silverhawk
For other peoples understanding, there's no such thing as too slowly with cost index. The speed from whatever cost index the company gives you is the most economic, no matter how much longer you spend in the air with the engines running. I imagine you where pointing out that a fixed cost index across all airfields and fleet that an airline operates isn't the correct way to utilise it though.
If all the operators worked out a pretty accurate cost index, we'd all be descending at a slow speed, maybe around 250kts.