"We added performance comparison with MongoDB. MongoDB is very slow on loading data (slide 59) - 8 minutes vs 76s,
seqscan speed is the same - about 1s, index scan is very fast - 1ms vs 17 ms with GIN fast-scan patch. But we managed to create new opclass (slides 61-62) for hstore using hashing of full-paths concatenated with values and got 0.6ms, which is faster than mongodb !"
GIN indexes are big, but Mongo's indexes are bigger than all other PostgreSQL indexes, and Mongo also requires much more space for the data.
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.
This is why MongoDB recommends all indices fit in memory