WebP offers incremental decoding

Source: https://developers.google.com/speed/webp/faq

Does WebP support progressive or interlaced display? WebP does not offer a progressive or interlaced decoding refresh in the JPEG or PNG sense. This is likely to put too much pressure on the CPU and memory of the decoding client as each refresh event involves a full pass through the decompression system.

On average, decoding a progressive JPEG image is equivalent to decoding the baseline one 3 times.

Alternatively, WebP offers incremental decoding, where all available incoming bytes of the bitstream are used to try and produce a displayable sample row as soon as possible. This both saves memory, CPU and re-paint effort on the client while providing visual cues about the download status. The incremental decoding feature is available through the Advanced Decoding API.