There is 0 difference in the speed of GC process. It does not matter if you are on H, L, O or don't have any status at all and applying from abroad. People are spreading this myth because L1-A and O-1 requirements are pretty close to EB1 so if you can get such a category of visa then you have a good chance to apply to EB1. But, again, if you fit these requirements already you can just apply to EB1 from any status (or lack of thereof).
Actually at least one cateogory, EB1-C, specifically applies to managers relocated from an overseas branch of the same company. An equivalently qualified manager on H1-B in the US is ineligible for that category.
Equivalently qualified manager on H-1B would have spent a year out of last 3 serving in such a position overseas so he would also be eligible. Alternatively, a L-1A manager who already spent more than 2 years in the US would be ineligible.
