I can't believe that most users here can't analyze the situation clearly. There is a big gap between the length of Minato's kunai to that of the tentacle as shown here
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If you'll compare the diameter of the tentacle to that of the kunai's length then it's very clear that there is a very big gap between them. How was he able to slice the tentacle? Even if Minato were to apply a great speed, the fact still remains that the deepness of the cut can be measured by the length of the kunai. Get my point?
Here is a better view:
Try to cut a 5 inch diameter papaya with a 1 inch knife. Can you cut it in half? or the deepness of the cut depends on the length of the blade? A 1 inch knife can produce a 1 inch deep cut. No matter how fast you are, you can't make a cut of 3 inches if you only have a 1 inch knife.
To sum it all, it's
possiblethat it is infused with a wind chakra. Why?
Here are the reasons:
1. Jiraiya said that Minato was a shrewd person, and
was not a man who would do something without reason.
take note of the bolded part
Why would he attempt to stab A knowing that A is using lightning shroud? Is that a trait of someone considered to be a shrewd person?
2. Wind>Lightning
We all know this one right? So, If we connect these two reasons, it will lead us to believe that Minato has wind affinity. It is
possiblethat he infused wind chakra because the cut is too deep. It is more than the length of his kunai.