I process uploaded images by php to save (after resize) by imagejpeg. As I explored, imagejpeg is the best php command to compress jpg images to reduce the file size. However, when I check my website by Google Page Speed, it says all of my images can be compressed 4-10%.
What is the common method to compress images to meet the Google standard?
Googles “standard” is the possible maximum to be expected by google. You need to use highly optimized image compressors that does nothing else than image compressing and therefore get a possible maximum best value here.
You can for example, open you jpeg file in a image editor like adobe photoshop and create the possible maximum best compression to be expected by you while having visual control. Highly recommended.
The GD library provides a standard conform jpeg compression which should match a library users expectation, but which might not satisfy a graphic designer (and/or google by 4-10%).