Fastly’s major advantage is near instant CDN config changes, where Akamai can take an hour to push a new cdn config.
Small changes only take a few minutes to roll out on Akamai's network. Another commenter talked about Fastly's POP count. Akamai has far more as can be seen here: https://www.akamai.com/uk/en/solutions/intelligent-platform/...
No doubt having so many POPs (some probably legacy and slower to update) likely extend the update time.