The partial response feature adds the ability to use the “fields” query parameter to request only relevant information. For example, if you are using the Calendar Data API, you can opt to select only the event title and time. This is a significant boon to performance, as the previous API forced applications to request all of the data.
via Google Data Protocol Features Make APIs Faster – Website Magazine – Website Magazine.
This seems like a “no duh”, but I would have thought with Google being so obsessed with performance, this would have been the case from the start. The only difficulty is that someone has to maintain relationships between related fields of data that need to be presented together. If you request one field only, there may be a minimum set of required fields always replied back, or related fields also replied back.