Faster, probably not by much, but it would have been a lot more power efficient. And yes, they have literally saved a few bucks per chip, you can use a wafer calculator and look up estimated prices for TSMC and Samsung and see for yourself. Of course all of this seems dumb and a mystery to people who don't anything about this, I don't blame you for believing that.
But then what's the alternative ? Say Samsung's node is fucking amazing as you think it is, the massive gap in efficiency between them and AMD must be explained somehow and the only other explanation is that instead of the node being garbage their architecture is garbage.
And by the way the "process made specifically for Nvidia" is actually the high performance version of Samsung's 8nm node, the same way 12nm was just TSMC's 16nm.