For Loop Significantly Increases Run Time of Script

Hello again! For this ode solver, I am trying to tell it to solve a system of equations for every time step in my for loop. The ode solver "ode15s" easily solves the trajectory on its own, however once I add a for loop, the script runs "infinitely" (Many hours). When I force stop the script, I see the trajectory plotted so far, but it shouldn't be taking as long as it is. Any reason why the for loop causes it to take much longer to solve?

The ode solver can solve this iteration of the code (WO for loop) very quickly:

[X,Y,Z] = meshgrid(-0.1:.01:0.1,-0.1:.01:0.1,-0.1:.01:0.1);
[Bx, By, Bz] = B_test();
Bfieldx = arrayfun(Bx,X,Y,Z);
Bfieldy = arrayfun(By,X,Y,Z);
Bfieldz = arrayfun(Bz,X,Y,Z);

icv = [0.05; 0.05; 0.; -1.7585E+7; -1.7585E+7; 0.]; %Format: [x; y; z; vz; vy; vz]

%Time Span (sec)
opts = odeset('MaxStep',1E-10);
tspan = [0 1E-8];

[T,W] = ode15s(@bdipuniodefun, tspan, icv);
[rownum,colnum] = size(W);

plot3(W(:,1), W(:,2), W(:,3), '-r', 'LineWidth',2,'color',[0 1 0])
xlabel 'x';
ylabel 'y';
zlabel 'z';
grid on
hold on

quiver3(X,Y,Z,Bfieldx,Bfieldy,Bfieldz)
axis equal
xlabel 'x'
ylabel 'y'
zlabel 'z'
title('Quiver Plot of B-Field Solution')
hold off

However when I add the for loop in this version, this is where it slows down:

[X,Y,Z] = meshgrid(-0.1:0.01:0.1,-0.1:0.01:0.1,-0.1:0.01:0.1);
[Bx, By, Bz] = B_test();
Bfieldx = arrayfun(Bx,X,Y,Z);
Bfieldy = arrayfun(By,X,Y,Z);
Bfieldz = arrayfun(Bz,X,Y,Z);

hold on
quiver3(X,Y,Z,Bfieldx,Bfieldy,Bfieldz)
xlabel 'x in m'
ylabel 'y in m'
zlabel 'z in m'
title('Particle Trajectory Solution with Collisions Densities')

%Initial position and velocity
spart = [0.05 0.05 0.];
vpart = [-1.7585E+7 -1.7585E+7 0.];

opts = odeset('MaxStep',1E-10);
tstep = 1E-10;
tfin = 1E-7;

for t = 0:tstep:tfin

      x = spart(1);
    y = spart(2);
    z = spart(3);
    vx = vpart(1);
    vy = vpart(2);
    vz = vpart(3);

      icv = [x; y; z; vx; vy; vz];
      tspan = [0 tstep];
      [T,W] = ode15s(@bdipuniodefun, tspan, icv);

  plot3(W(:,1), W(:,2), W(:,3), '-r', 'LineWidth',2,'color',[randi(0:1) randi(0:1) randi(0:1)])
  xlabel 'x';
  ylabel 'y';
  zlabel 'z';
    grid on

      vparts(1) = W(rownum,4);
    vparts(2) = W(rownum,5);
    vparts(3) = W(rownum,6);
    sparts(1) = W(rownum,1);
    sparts(2) = W(rownum,2);
    sparts(3) = W(rownum,3);

    end

hold off