PageSpeed Optimization Filters
This optimization recompresses jpegs in two different ways, depending on whether the input jpeg's quality higher or lower than PageSpeed's default output quality, which is ImageRecompressionQuality, or JpegRecompressionQuality if set. If the input image is higher quality than PageSpeed is set to produce then
recompress_jpeg will recompress the image to the lower quality setting. Otherwise PageSpeed will attempt to improve the compression of the image but won't reduce its quality.
pagespeed EnableFilters recompress_jpeg
pagespeed DisableFilters recompress_jpeg