A few things that could be hacked on.

Remove httpsig key fetching from the input path.
	If the key is known, check signature and reply 200 or 401.
	If the key is unknown, immediately reply 202 and process later.

The gob encoding for backend rpc uses more memory than needed.
	A custom encoding could reduce allocations.
	Maybe the backend could fetch the data itself.

Several columns and indices have potentially long shared prefixes.
	These could be stored rearranged, perhaps with last four bytes prepended.
	https://here/there/9876 -> 9876https://here/there/9876