Never use wp smush plugin, sometime when we make big sites this plugin fail to decrease size of images only work in start so this can be effect at our ranking always use GIMP software to decrease size of your images. to make safe your image size in speed.
Download and Open GIMP
Open Image You Want to Upload on Your Site
Click Export as Jgp or Png
Decrease quality to 35%
and save it.
This will decrease your size of image under 20KB and keep safe your image size issues on Webpage speed.
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