Functional vs simple code

[ixid]

Without optimization the range and algorithm method takes about 
10 times as long as the simple code below it, with no array 
bounds checking and optimization it takes six times as long. Why 
is the difference so huge? I'd expect a moderate overhead but 
that's a big difference.

module main;
import std.stdio, std.algorithm, std.range, std.datetime;
enum MAX = 10_000_000UL;

void main() {
        StopWatch sw;
        sw.start;

        auto sum1 = MAX.iota.map!(x => x * x).reduce!"a + b";

        sw.peek.msecs.writeln("msecs");
        sum1.writeln;
        sw.start;

        ulong sum2 = 0;
        foreach(i;0..MAX)
                sum2 += i * i;

        sw.peek.msecs.writeln("msecs");

        sum2.writeln;
}