Mollies are brackish water fish and when acclimating any fish you should take a little time. As you found out you can usually take a mollie and throw it from across the room into a tank full of salt water and it will probably live for five years. Now, stop cringing, I didn't say to do that, but a mollie is a very sturdy fish and easy to keep in almost any water. I would take at least two hours to acclimate it as I have done it dozens of times with brackish fish like mollies and bumble bee gobies which are better looking and will live a couple of years in full salt water. The fishes kidney is responsible for stabilizing the salinity inside the fish and some fish can do that relatively fast but slower would be better for the fish. Braskish fish normally swim from full salt to fresh water such as when they go from the ocean up into a river. Those fish were "designed" to be able to do that. The tide pool where I collect is full salt at high tide but at low tide it is almost full fresh because it is from from a lake. Bluefish, which are not even brackish fish swim back and forth with no obvious problems.
