1. What does DirectX 12 bring?
2. How does the performance affect a GT 1030?
With DX12, the CPU resources are used better. With a GT 1030, you hardly notice it.
1. Is simply ne rendering api.
2. What are you getting at? You may have 1fps more if you are lucky. The GT 1030 remains a very bad gaming card.
And how about an RTX 3080?
Depending on the game, you have + - 1-10% fps.