Search engines take into account the loading time of a website in order to classify sites or zenphoto does not exploit optimally the fast loading. For example, it would be interesting to add:

  - Mod_Gzip
  - Expires headers

Can you integrate them in the next version of zenphoto.

You can enable both htaccess if you need them.

Also note other than Piwigo you apparently seem to come from Zenphoto does not pre-generate image sizes on uploading but on request. This can slow down initial loading but happens once. You can also precache images using the cacheManager plugin. Additionally there is a html cache plugin. See the user guide about caching.

The html cache plugin is interesting but the plugin puts all the images in cache is what can be put only the images consulted them to reduce the load on the server. Because if the site contains for example 10000 photos the plugin will put a certain time and saturate the server.

The html cache plugin does not put any images anywhere actually. Did you read the user guide article about how image caching works?