Replication of images from your server happens on-demand, when the first user visits a page. New images are detected, copied to the Sirv server, then optimized and served. Copying typically takes 1-2 seconds, depending on how quickly your server responds. Alternatively, you can sync your entire library in one go by clicking the Sync button in your plugin settings. It will prevent any delay for the first visitor.
Sirv delivers the most optimal image format for each image. The smallest image is often WebP, so Sirv automatically generates these amazingly small images to browsers which support them (Chrome and Opera). Other browsers receive a JPEG or PNG, depending on the image.