It's possible that even when slowing down the roundabout still ends up being more efficient, since you don't have as much "stop/go" action going on

This is generally how it works in the UK, except for newbies who tend to always stop completely rather than try to time the merge where possible... I've heard that this is actually taught in advanced driving in the UK (to maintain flow when possible, time merges etc). Frankly there are some roundabouts in cities at rush hour which are frightening for new drivers and you just can't join unless you have good timing and can get up to speed quickly (or get a lucky smooth merge).