Nothing new. We already knew to enter Sage Mode takes long to kneed chakra, like we also knew without a partner you can't maintain it for long(Naruto could only maintain it for 5minutes for example)
It is and it isnt something new.
To enter it initially it takes a bit of time being still. It was never said that you needed very long.
Initially naruto needed a little while or so to enter sage mode.
Then after training(in the war) he was able to enter it in a shorter time.
Then during the juubi arc, he was able to enter it in an even shorter time.
Then he was able to do it nearly instantly with BSM.
So what im getting at, is it only takes a long time to enter sage mode, if you are a beginner. If you practice and train, you can enter it much quicker(to the point it almost isnt a handicap to have to enter it.)
The same thing goes to maintaining it. If you are a beginner, then you cant maintain it long. You can maintain it longer the more you train.
And then there is a final factor. Talent.
Minato had some talent(as such he was able to have perfect sage mode(or i should say perfect balanced sage mode).
However either due to haveing just learnt sage mode, or due to skill with it, he was able to maintain sage mode long, and it takes him longer to enter it.
But this isnt some completely new territory. But it did give us more insight about sage mode.(expanding what we already knew)