the more things you render the lower the framerate will get. that you can run from.

as for rendering or adding a cubbie every n'th second its up to you to change the count of cubbies to render.

this will increase number of cubbies to render every 3 seconds.

// @ mainloop
float time = (millis() - currTime) * 0.001;
if( time >= 3.0 )
{
currTime = millis();
counter ++;
}

//in void draw()
for (int i=0; i<counter; i++)
{
   pushMatrix();
   translate(x[i], y[i], z[i]);
   noStroke();
   c[i].create(quadBG[i]);
   x[i]+=xSpeed[i];
   y[i]+=ySpeed[i];
   z[i]+=zSpeed[i];
   popMatrix();
}