If the noncitizen spouse is outside the US, K3 takes almost the same amount of time as an immigrant visa, so overall the path to the green card is faster with an immigrant visa because they would obtain permanent resident status immediately upon arrival in the US with that visa, whereas with K3 they still have to follow the adjustment process to become a PR.
Now as far as immigrant visa vs. adjustment of status is concerned, in general it is around the same time either way, but it is country-dependent because some consulates are slower than others. However, adjustment of status can be problematic if the noncitizen spouse enters with a nonimmigrant visa like B2 which does not allow immigrant intent; you risk being denied for misrepresenting intent at the port of entry. On the other hand, the consular processing route requires obtaining police certificates from every country the noncitizen has lived in since the age of 16, which could be a big problem for somebody who has lived in many countries.