the scrubbed version "floating around" has the update partition removed and the regular partition shuffled up to fill its space, which is poor practice. this causes loading issues playing from a burnt version of the scrubbed disc. the game will also do brief loading pauses when played from that disc, mid-firefight.
using one without the update partition raped, or playing from the usb loader, will likely fix the issue. IF you are already doing one of those, I read wrong, and so I don't know what would cause your issue.
when I beta-tested the game and loading was not ever a problem at that time, playing a devkit w hdd.