Images are usually the heaviest part of a shop page and the easiest performance win.
Upload sensibly
Decide the largest size you will ever display – usually the zoom image – and upload at that. Uploading 6000-pixel camera files is wasted space and slows every upload, backup and page.
WooCommerce generates several sizes
For each product image WooCommerce creates a thumbnail, a catalogue image and a single-product image, on top of the WordPress sizes and anything your theme adds. Ten sizes per image is not unusual. That multiplies both disk usage and inode count – see Why you have run out of inodes.
When images look wrong after a theme change
Changing theme changes the registered image sizes, but existing images are not re-cut automatically. The result is blurry or stretched product images. A regeneration plugin re-cuts them from the originals.
Regeneration is heavy. On a large catalogue it will use a great deal of CPU and disk. Run it at a quiet time, in batches, and not during a sale.
Compression
A compression plugin typically reclaims a large share of image weight with no visible difference. Keep the originals until you are happy with the result.
Lazy loading
A category page showing forty products loads forty images. Lazy loading defers the ones below the fold. Modern WordPress does this by default; check your theme has not disabled it.
A word on formats
Modern formats such as WebP are considerably smaller. Most compression plugins can serve them automatically with a fallback for older browsers.
