It takes a non-0 amount of time to load large indices into memory (upwards of 5 seconds, depending on your choice of disks and arrays), and to iterate through large indices once they are in memory.
Such large indices also limit how many of them can be stored in memory at the same time - making transactions against those indexes slower over time if the indices have to be loaded and unloaded from memory frequently.
You should always choose indexes with care, because indexes not only cost space, they cost time. And as you said, Time is expensive.
> It takes a non-0 amount of time to load large indices into memory (upwards of 5 seconds, depending on your choice of disks and arrays), and to iterate through large indices once they are in memory.
This is why MongoDB recommends all indices fit in memory
Such large indices also limit how many of them can be stored in memory at the same time - making transactions against those indexes slower over time if the indices have to be loaded and unloaded from memory frequently.
You should always choose indexes with care, because indexes not only cost space, they cost time. And as you said, Time is expensive.