Insurance premiums are indeed growing more slowly. That's because they are instead increasing deductibles and decreasing coverage. What has happened is they've found the maximum employers are willing to pay, so they are increasing costs elsewhere. Further none of the premium charts I see appear to show the same health coverage, they are instead an average of all premiums. Of course with millions of people previously not insured who must now choose to insure or face higher taxes, they will choose cheap health insurance plans with lower premiums. If you don't control for coverage or deductibles you are most certainly going to find the average premium going down.That does sound, in part, like some unusual circumstances. But insurance rates have always increased. According to what I have read, they increased faster every year before ACA.
I haven't found a good graph of out of pocket medical costs, but I'd expect that to show the significant increase as well.