Board Thread:Versus Debates/@comment-45235418-20200316185119/@comment-4080775-20200410200302

Minato can enter Sage Mode instantly. He displayed this when he went in to attack Madara, entering sage mode after the kunai was thrown, meaning he was casually gathering natural energy before he went in to attack Madara.

Also from th Sage Mode page:
 * "If the user is also a jinchūriki that has achieved harmony with their tailed beast, they can drastically speed up their ability to gather natural energy in tailed beast mode to the point of only needing a few seconds to fully charge or recharge their sage chakra."

Minato has achieved harmony with his half of Kurama, and as such, should be able to fully recharge his sage chakra in a few seconds. Minato takes this.