parallel is slower than serial

[Yura via Digitalmars-d-learn]

Dear All,

I am trying to make a simple code run in parallel. The parallel 
version works, and gives the same number as serial albeit slower.

First, the parallel features I am using:

import core.thread: Thread;
import std.range;
import std.parallelism:parallel;
import std.parallelism:taskPool;
import std.parallelism:totalCPUs;

// Then, I have an array of structures

shared Sphere [] dot;

// Each Sphere is

struct Sphere {
  string El;
  double x;
  double y;
  double z;
  double S;
  double Z;
  double V;
}

// Then for each Sphere, i.e. dot[i]
// I need to do some arithmetics with itself and other dots
// I have only parallelized the inner loop, i is fixed.

// parallel loop
auto I = std.range.iota(0,dot.length);
shared double [] Ai;
Ai.length = dot.length;
foreach (j;parallel(I)) {
  Ai[j] = GETAij (i, j, dot[i], dot[j]);
}

for (auto j=0;j<Ai.length;j++) {
  A = A ~ Ai[j];
}

// the function GETAij

// this is the function to calculate Aij cells
// in parallel

double GETAij (ulong i, ulong j, Sphere dot_i, Sphere dot_j) {
  double Aij;
  if (i == j) {
    Aij = 1.0694*pow((4*pi/dot_i.S),0.5);
  }
  else {
    Aij = 1/(distDD(dot_i,dot_j));
  }
  return Aij;
}

double distDD (Sphere A, Sphere B) {
  double dx2 = (A.x-B.x)*(A.x-B.x);
  double dy2 = (A.y-B.y)*(A.y-B.y);
  double dz2 = (A.z-B.z)*(A.z-B.z);
  double d = pow((dx2 + dy2 + dz2),0.5);
  return d;
}

What I am doing wrong? Any advanced options for the ldc2 
compiler? Many thanks in advance!