The library registry sends OPDS 2 documents that would compress by about 50%, saving bandwidth. We can compress the documents with gzip if the client supports Transfer-Encoding – and if it doesn't, it ought to.

The library registry can also instruct the client how long to cache these documents.

It's possible that we could also send ETag and Last-Modified headers to allow the use of conditional GET. This might not be a good long-term solution, though, since in the medium term the input into the registry endpoint is going to have a latitude and longitude as input. This will make caching effectively useless unless we round off the input position.