Progressive JPEGs: Enhancing Perceived Load Times
Why progressive scans are superior to baseline rendering for slower network connections.
DM
Dave Miller
Contributing Author · Squoosh Next BlogBaseline JPEGs decode and render top-to-bottom: the user sees a sliver of image that grows downward as bytes arrive. Progressive JPEGs encode multiple scans of increasing quality — the first scan delivers a blurry full-frame preview in as few as 2KB, and subsequent scans refine detail progressively. This means users perceive the image almost immediately, even on slow connections, while the browser continues downloading.
MozJPEG implements progressive JPEG encoding with trellis quantization optimization applied across all scans, producing files that are typically 5–10% smaller than equivalent baseline JPEGs while delivering a dramatically better perceived performance story on mobile networks.
Key Takeaways
Baseline JPEGs decode and render top-to-bottom: the user sees a sliver of image that grows downward as bytes arrive.
Progressive JPEGs encode multiple scans of increasing quality — the first scan delivers a blurry full-frame preview in as few as 2KB, and subsequent scans refine detail progressively.
This means users perceive the image almost immediately, even on slow connections, while the browser continues downloading.
MozJPEG implements progressive JPEG encoding with trellis quantization optimization applied across all scans, producing files that are typically 5–10% smaller than equivalent baseline JPEGs while delivering a dramatically better perceived performance story on mobile networks.
Try It in the Workspace
Everything discussed in this article can be tested directly in Squoosh Next — no sign-up, no upload, 100% client-side.