As a Wix Web Designer, I find that optimizing your images to their used size IS very helpful with page speed, Wix doesn't do the best job at optimizing the images for best page load time.
Below is a screenshot of a quick example from Google's Page Speed Insights tool (developers.google.com/speed/pagespeed/insights) that shows a before/after score.
I increased my Page Speed score by simply reducing the size and optimizing 6 small images on my homepage. This was just a simple change, but my score improved by 3 points (65 to 68/100). I will optimize the rest of the images it suggests as well to help improve my page speed even more.
Wix Image Size Recommendations:
Background/Strip Large Full-Width Images; 1920 pixels wide
Uploaded images should be at least 1920 wide for full-width scaling images such as in Strips or backgrounds. Wix auto resizes images on the page depending on the window and device, but it's still wise to keep them relatively smaller at 1920 max pixels wide by whatever height for best page speed.
Smaller Images – size to the size used
If you are using smaller images on the page, it's best to size them at the size used – but it's also good to upload larger versions as well for future use at any size if needed. So you can make two image sizes, one large one (1920 wide x height), and a second one that is optimized for the size used on the page (keep in mind to double your resolution so if the image is showing up at 400x200 pixels high on your page, size it at 800x400 pixels so it will display nicely on high resolution monitors/devices).
Use JPG Format:
For optimization, it is preferable to upload it in JPG format – PNG formats should only be used if your image requires a transparency layer, and they DO increase load time if the files are large, so use these sparingly and only if needed for transparency.
JPG Quality: Use JPG quality of 80 - 90 (for a scale from 0 - 100).