JIT_New () is a helper function inside the CLR that runs whenever you create a new object in code with a new statement. It simply allocates memory from the collected garbage heap and calls the class constructor. Or, in other words, he implements the Opcodes.Newobj IL team. His name is a bit confusing; it has nothing to do with jittering your code. Just a helper function that jitter knows about, it compiles a call to that helper function directly into the generated machine code. JIT_Newarr1 () will be the other you come across, it allocates an array.
I don't know PerfView, note that the runtime for JIT_New () may include the time it takes for garbage collectors to execute. What happens when the gen # 0 heap is full when JIT_New () is run. Which explains the large percentage, JIT_New () is otherwise very fast. There is nothing you can do about it; this is the correction of the overhead in any managed program.