Why does my libigl code using knn take forever to complete?

Question

The following code loads a mesh off file, creates a vector of set of random points from the mesh. Then it finds the nearest points on the mesh for each of these sets of random points using knn. I found that the code is stuck in the knn step forever. This is for a mesh with 50000 points and 100000 faces. How to speed it up? Is there any error in my code as given below:

#include <igl/read_triangle_mesh.h>
#include <igl/octree.h>
#include <igl/knn.h>
#include <igl/random_points_on_mesh.h>
#include <igl/opengl/glfw/Viewer.h>
#include <igl/get_seconds.h>
#include <iostream>
#include <cstdlib>
#include <vector>

int main(int argc, char *argv[])
{

  Eigen::MatrixXd V;
  Eigen::MatrixXi F;
  igl::read_triangle_mesh(argv[1],V,F);
  // Sample mesh for point cloud

  std::cout << "no. of points, faces "<< V.rows()<<"," <<F.rows()<< std::endl;

  std::vector<Eigen::MatrixXd> vec_pts;

  for (int i =0;i<10;i++)
  {
    Eigen::MatrixXd P;
    Eigen::VectorXi I;
    Eigen::MatrixXd B;
    igl::random_points_on_mesh(25,V,F,B,I,P);
    vec_pts.push_back(P);
  }

  std::cout  << "points created "<< std::endl;

  // Build octree
  std::vector<std::vector<int > > O_PI;
  Eigen::MatrixXi O_CH;
  Eigen::MatrixXd O_CN;
  Eigen::VectorXd O_W;

  igl::octree(V,O_PI,O_CH,O_CN,O_W);
  std::cout  << "octree created "<< std::endl;


  for (int i=0;i< vec_pts.size();i++)
  {
    Eigen::MatrixXd query = vec_pts[i];
    Eigen::VectorXi I;
    const double t_before = igl::get_seconds();
    igl::knn(query,1,O_PI,O_CH,O_CN,O_W,I);
    const double t_after = igl::get_seconds();
    std::cout  << "time for knn query in sec "<< t_after - t_before << std::endl;
  }
}