Yes you should find that images in the UI are fast-loading JPEGs. Photonix has a thumbnailing service built in which produces smaller versions of photos for display. You won't tend to see the original files unless you click the download icon. The most frequently seen JPEGs are:
256x256px, 'cover', 50% quality, ~5-10KB
3840x3840px, 'contain', 75% quality, ~500KB-1MB
There is naturally a trade-off between size and quality. On our photo detail screen we load a slightly larger images than you may expect but this is because we still want to show a reasonable level of detail if the user tries to zoom in.
With relation to raw files, a full-size JPEG is created of all of these which is then used to create smaller thumbnails from.