How does delta compression help?

Ably supports deltas on messages which can reduce the encoded size of each message when the difference between successive message payloads is small e.g. subscribing to a sports game where the score is updated but no other part of the message data changes.

Deltas mean that the bandwidth on any deltas-enabled connection can be lower, and the total bandwidth is lower (translating to lower message costs overall).

It doesn't however overcome the per-message size limit as all messages need to sent in full because some subscribers may not subscribe in deltas mode