Ostensibly, a main reason for separate foreground/background builds is that foreground index builds are faster, at the cost of blocking the server [1]. However, we have observed both in production and in synthetic benchmarks, that foreground index builds are often much slower on large collections.

In a synthetic benchmark, building a trivial index on a collection with about 6M records, each about 1k large, I saw the following numbers:

[2014-03-21 00:05:10,042 22310|INFO] Done. items=6250000 fg=145.1 bg=94.7
[2014-03-21 00:09:01,545 22310|INFO] Done. items=6250000 fg=135.7 bg=95.4
[2014-03-21 00:12:43,822 22310|INFO] Done. items=6250000 fg=125.1 bg=96.9
[2014-03-21 00:16:25,450 22310|INFO] Done. items=6250000 fg=125.6 bg=95.8
[2014-03-21 00:20:00,567 22310|INFO] Done. items=6250000 fg=122.5 bg=92.3

The "fg=" number is seconds to build an index in the foreground, and "bg=" is for a background build. The requested indexes are identical, and are dropped each time.

I don't have hard numbers right now, but experience in production suggests that the difference only gets worse as the collection gets even bigger.

[1] http://docs.mongodb.org/manual/tutorial/build-indexes-in-the-background/