While the product images are stored remotely (AliExpress) we have no control over optimisation on the server side. Sellers upload their images and I imagine AliExpress has their own Optimising Methods - However, there are other options available.
It is time consuming but worth taking the time to use some of the suggestions here on this page, it has good advice
Click Me on image optimisation.
Some stuff you have no choice to import to your own server, in which case the first post recommending Smush is great. Personally, I've been considering using a VPS as a media server so I can optimise before it gets pulled in to my site.
Hope this helps