ETags, and the conditional request nobody sends

An ETag costs nothing to emit and saves the response body only if clients actually send the conditional header back.

GET /api/orders/8814
→ 200 OK
  ETag: "a41f2b8"

GET /api/orders/8814
  If-None-Match: "a41f2b8"
→ 304 Not Modified      # no body

# and the other direction, which is the underused one:
PUT /api/orders/8814
  If-Match: "a41f2b8"
→ 412 Precondition Failed   # somebody else changed it

The write side is where ETag earns its place: If-Match gives optimistic concurrency control for free, so two clients editing the same resource produce a 412 instead of a silent overwrite. The read side depends on clients being well behaved and mostly they are not, so the saving is smaller than expected. Generating the tag from a version column rather than hashing the body is cheaper and avoids rendering the response to find out it was not needed.