Tell GIMP not to include the EXIF and XMP data (which doesn't get any smaller when you decrease the quality). Then tell it to use 4:2:0 chroma subsampling, and

Tell GIMP not to include the EXIF and XMP data (which doesn't get any smaller when you decrease the quality). Then tell it to use 4:2:0 chroma subsampling, and set the quality to 24 or so.