Per my runs of tests1-4 on develop with 500m, 1b and 2b items, we see that the compact operation takes about 140x longer with 2b items than with 500m or 1b. (This was run on AWS SSD, production build, changeset fbeb5e2 from May 29, although seen from a Feb changeset too.)

1 billion items completes compact in 108 seconds. 2 billion items takes 14417 seconds.