Writing Album Art to FLAC very slow over LAN

I have all my CD's ripped to FLAC file per album. Each FLAC file varies from 200MB to over 500MB. They are all stored on my NAS which has a Gigabit connection. I think because the NAS is rather full, it writes at a speed of around 3MB/s whilst it can read at 20MB/s.

I'm attempting to Album Art to each FLAC and over the network, MP3Tag can take up to 20 minute to write the image file into the tag for just one album. If the file is stored locally, it's just a second or two.

Is this expected? Is there an alternative that would be faster.

If not, I may transfer all files to my PC one night, write the images to them, then transfer them back to the NAS the next night.

I believe the reason it takes so long is that there isn't enough padding at the beginning of my FLAC files to accomodate the amount of data required to store the image. As a result, the entire file needs to be rewritten.

I'm going to copy all my FLAC files to my main PC, then use metaflac to add 1MB of padding, to ensure that I can add more metadata in future without hitting this problem again.