Because Unicode

Because Unicode

Never do that

var vectorMax = Vector<int>.Zero; but var vectorMax = Vector<int>(int.MinValue); should be, I think

var blockVector = new Vector<int>(nums, blockIndex); copy memory

vectorMax = Vector.Max(vectorMax, blockVector); copy memory

2 memory coping. Is it faster then classic loop?

if (items == null || items.Length == 0) C# compiler makes check on null them self, so if (items.Length == 0) is enought.
max = items[i].CompareTo(max) > 0 ? items[i] : max; is slower then if (max.CompareTo(items[i]) < 0) max = items[i];

I love var more then T because code can be copied and paste to another place without changing.