Understanding the “xtag-sp-debug-reason” header

When checking your site, you might encounter a response header called xtag-sp-debug-reason. This header provides diagnostic information from ShortPixel’s CDN about why an image might not be served or optimized as expected.

Below are the most common values for this header and what they mean:

Processing queue temporarily exceeded for this host #

This means our servers have temporarily throttled the optimization queue for your domain.

At ShortPixel, we use an internal queue throttling mechanism to manage resources fairly across all users. If a high-traffic website sends many image optimization requests at once, the system slows down processing to prevent overload that could affect the optimization speed for other customers.

The throttling is based on a point system:

• 1 JPG = 1 point
• 1 PNG or WebP = 5 points
• 1 AVIF = 10 points

In other words, processing a PNG or WebP is 5 times more expensive than processing a normal JPG and an AVIF is 10 times more expensive, because they are more resource-intensive to process. And that’s why they count for more points.

This is not something you can improve as a customer. These are technical limitations because the servers need much more time to process a PNG/WebP/AVIF than a JPG.

If your site tries to process, for example, 100 JPGs or 10 AVIFs at the same time, the system may briefly pause your queue for 5–10 seconds while processing catches up. This happens automatically and is usually invisible to visitors.

Importantly, this only affects the first request. Once an image is optimized, it’s served directly from the CDN without delay—even during high traffic.

URL banned redirect #

This value indicates that our servers were temporarily blocked from downloading images from your site.

Here’s what typically happens:

1. Our system tries to download your images via a proxy server.
2. If the proxy is blocked (often due to a firewall), a direct connection is attempted.
3. If many workers fail to access your images—due to firewall or security settings—a temporary domain-level ban is triggered.
4. This results in all images returning a 307 redirect for about 30 minutes.
5. After that, the system tries again, but if the issue persists, the cycle repeats.

Solution: If you see this header value, we recommend whitelisting our IP addresses on your firewall. This will allow our servers to access your images and prevent these temporary bans.