Two solutions with min max values - which should be slower?
To answer your question: those solutions are identical performance-wise. There is no "magic" in Min or Max methods: they do the same thing you do in your first solution - compare the two values using if operator. There is no significant performance cost for a method call or for an extra assignment - those operations are so cheap, that you won't solve any real-life performance issue with this kind of optimization. If you will become obsessed with those small things - it will have a negative effect on your code quality. So, instead of asking, which is faster, you should ask yourself which is more clean, readable and OOP-ish. I would go with second solution.

If you want to really speed things up - you should first identify the real problem. In your example - it is the number of iterations, not the number of assignments. So the faster solution would probably involve multi-threading and iterating through different parts of your array simultaneously.