I've been using Mp3tag for several years, and for as long as I can remember, writing 100 KB album art to FLAC files has been very slow if the files must be rewritten. It might take a minute or two to update a dozen files 20-40 MB each. This is happening again with Mp3tag 2.57 on my new build, which is an i5 4670 with 16 GB RAM. While the source and destination drives are the same WD green drive, this is still inexplicably slow and far slower than simply duplicating the files. It is actually far faster for me to load the FLAC files into EZ CD Audio Converter, add the album art, and transcode to ALAC. For example, I just now added the same 95 KB album art to these two sets of files lacking album art, all of which have been on my hard drive for several months so there is no prior caching to influence the results:
EZ CD Audio Converter, 12 FLAC files, 345 MB total, transcoding to ALAC + adding album art, 1 thread: 17 seconds
Mp3tag, 12 FLAC files, 264 MB total: 73 seconds
As it often does, Mp3tag got through the first six files pretty quickly, but then it bogged down on file seven and after, which took up most of the time. Any ideas?
ETA: The OS is Windows 7 x64.