Why is my code starts fast and then eventually slows down?

Well, I have a problem with this script that take video from my webcam and process it the image for detect some objects with basic colors... it's hardly fast, at first, but after some seconds it gets so slowly... can you help me?

The code is the next, and it's divided by principal function and other that

process..

% Script para detectar los colores rojo, azul y amarillo.
% Francisco Mendoza Ruiz, estudiante de ingenierÌa electrÛnica
% de la universidad autÛnma del Carmen (UNACAR).

%%-------------------------PRINCIPAL--------------------------------------

function vid_tiem_ejec(num_tomas)

vid = videoinput( 'macvideo' , 1 );

set( vid , 'ReturnedColorSpace' , 'RGB' );

triggerconfig( vid , 'manual' );

start(vid);

for i=1:num_tomas

      I = getsnapshot(vid);
      I = imresize( I , 0.5);
      colores(I)

end

stop(vid);

clear all

close all

end

%%---------------------------IMAGE PROCES PER FRAME---------------------------------

function colores(I);

r = I( : , : , 1);
g = I( : , : , 2);
b = I( : , : , 3);

rojo = r-g-b;
verd = g-b-r;
azul = b-r-g;
amar = g-r/2-b/2;

rojo = rojo > 35;
verd = verd > 35;
azul = azul > 20;
amar = amar > 20;

rojo = bwareaopen( rojo , 700 );
verd = bwareaopen( verd , 700 );
azul = bwareaopen( azul , 700 );
amar = bwareaopen( amar , 700 );

det_roj = regionprops ( rojo , 'Centroid' , 'Area' , 'BoundingBox' );
det_ver = regionprops ( verd , 'Centroid' , 'Area' , 'BoundingBox' );
det_azu = regionprops ( azul , 'Centroid' , 'Area' , 'BoundingBox' );
det_amar = regionprops ( amar , 'Centroid' , 'Area' , 'BoundingBox'  );

[ id_roj , ~ ] = size(det_roj);
[ id_ver , ~ ] = size(det_ver);
[ id_azu , ~ ] = size(det_azu);
[ id_amar , ~ ] = size(det_amar);

num_obj = id_roj + id_ver + id_azu + id_amar;

pts_roj = zeros ( 1 , id_roj );
pts_ver = zeros ( 1 , id_ver );
pts_azu = zeros ( 1 , id_azu );
pts_amar = zeros ( 1 , id_amar );

if ~isempty(id_roj) % CondiciÛn de existencia del objeto.
  for i=1:id_roj

      pts_rojx(i) = det_roj(i).Centroid(1);
      pts_rojy(i) = det_roj(i).Centroid(2);

    end
  end

if ~isempty(id_ver)
  for i=1:id_ver

      pts_verx(i) = det_ver(i).Centroid(1);
      pts_very(i) = det_ver(i).Centroid(2);

    end
  end

if ~isempty(id_azu)
  for i=1:id_azu

      pts_azux(i) = det_azu(i).Centroid(1);
      pts_azuy(i) = det_azu(i).Centroid(2);

    end
  end

if ~isempty(id_amar)
  for i=1:id_amar

      pts_amarx(i) = det_amar(i).Centroid(1);
      pts_amary(i) = det_amar(i).Centroid(2);

    end
  end

imshow(I);

if ~isempty(id_roj) && id_roj(1)~=0 
  hold on;            
  plot( pts_rojx , pts_rojy, '-b+' );

    for i=1:id_roj
      rectangle( 'Position' , det_roj(i).BoundingBox, 'Edge' , 'g' , 'LineWidth' , 2);
    end

end

if ~isempty(id_azu) && id_azu(1)~=0
  hold on;
  plot( pts_azux , pts_azuy, '-r+' );

    for i=1:id_azu
      rectangle( 'Position' , det_azu(i).BoundingBox, 'Edge' , 'r' , 'LineWidth' , 2);
    end

end

if ~isempty(id_amar) && id_amar(1)~=0
  hold on;
  plot( pts_amarx , pts_amary, '-m+' );

    for i=1:id_amar
      rectangle( 'Position' , det_amar(i).BoundingBox, 'Edge' , 'm' , 'LineWidth' , 2);
    end

end

if ~isempty(id_ver) && id_ver(1)~=0
  hold on;
  plot( pts_verx , pts_very, '-y+' );

    for i=1:id_ver
      rectangle( 'Position' , det_ver(i).BoundingBox, 'Edge' , 'y' , 'LineWidth' , 2);
    end

end

drawnow;

end

%%____________________________________________________________

This is a preview of the output:

I'll be grateful because your support. Thank you all.