50% is extremely optimistic between HEIF and a good JPEG encoder (mozjpeg/guetzli).
Guetzli usually produces very slightly better results than mozjpeg but is several hundred times slower. The 20-40% comparision is vs libjpeg.
Guetzli usually produces very slightly better results than mozjpeg but is several hundred times slower. The 20-40% comparision is vs libjpeg.
In my experience, Guetzli is a lot slowly than mozjpeg, but tops up at much higher space savings when the conditions are right.