MATLAB: PixelLabelDatastore read() function gets progressively slower

Question

The following code proceeds quickly (200 images/sec) until around the 12,000th read at which point the read time increases by roughly 10% per 200 reads flattening out at about 20 images/sec.

There are 1.4×10^6 images in the directory.

At the initial rate of 200/sec execution would take 2 hours to complete.

At the rate read settles to it would take 20 hours.

pxds = pixelLabelDatastore('directory', classes, LabelIDs);
parfor r = 1:length(pxds.Files);
    img_holder{r} = uint8(ones(200,200));
end
counter = 1;
while hasdata(pxds);
    img_holder{counter} = uint8(read(pxds));
    counter = counter + 1;
end

Answer 1

I ended up making an imagedatastore of the labelled images and then creating the categorical images and imwrite()'ing them in a parfor loop. It progressed at 500 images per second and did not slow down.

function y = convert_categorical(imds); 
      parfor r = 1:length(imds.Files); 
          [img, info] = imds.readimage(r); 
          [~, filename, ext] = fileparts(info.Filename); 
          if exist([path filename ext], 'file') == 0; 
              q = uint8(ones(227,227)); 
              q(img(:,:,1) == 0 & img(:,:,2) == 0 & img(:,:,3) == 255) = 1; 
              q(img(:,:,1) == 255 & img(:,:,2) == 255 & img(:,:,3) == 255) = 2; 
              q(img(:,:,1) == 0 & img(:,:,2) == 255 & img(:,:,3) == 0) = 3; 
              imwrite(q,[path filename ext]); 
          end 
      end 
end