It takes from 5 (shortest) to almost 40 (longest) minutes for `bitcoin-cli gettxoutsetinfo` to return an _empty_ dict. I noticed this first whilst configuring the Statoshi branch of Bitcoin Core, but went back to install Bitcoin Core (git clone -b v0.11.2 https://github.com/bitcoin/bitcoin.git) to find the same results.

```
20:10 pi@raspinode:~/bin/bitcoin.orig $ bitcoin-cli gettxoutsetinfo
{
}
20:12 pi@raspinode:~/bin/bitcoin.orig $ bitcoin-cli getinfo
{
    "version" : 110200,
    "protocolversion" : 70002,
    "blocks" : 393193,
    "timeoffset" : 0,
    "connections" : 12,
    "proxy" : "",
    "difficulty" : 113354299801.47113037,
    "testnet" : false,
    "relayfee" : 0.00005000,
    "errors" : ""
}
20:17 pi@raspinode:~/bin/bitcoin.orig $
```

Raspberry Pi 2 - 1GB memory - 4 core CPU 900MHz - Blockchain on SanDisk Ultra Android microSDXC 128GB up to 80 MB/Sec Class 10

All other RPC calls through bitcoin-cli are answered promptly and correctly.
See also issue on jlopp/statoshi https://github.com/jlopp/statoshi/issues/19#issuecomment-171350739

{
    if (fHelp || params.size() > 1)
        throw runtime_error(
            "getrawmempool ( verbose )\n"
            "\nReturns all transaction ids in memory pool as a json array of string transaction ids.\n"
            "\nArguments:\n"
            "1. verbose           (boolean, optional, default=false) true for a json object, false for array of transaction ids\n"
            "\nResult: (for verbose = false):\n"
            "[                     (json array of string)\n"
            "  \"transactionid\"     (string) The transaction id\n"
            "  ,...\n"
            "]\n"
            "\nResult: (for verbose = true):\n"
            "{                           (json object)\n"
            "  \"transactionid\" : {       (json object)\n"
            + EntryDescriptionString()
            + "  }, ...\n"
            "}\n"
            "\nExamples\n"
            + HelpExampleCli("getrawmempool", "true")
            + HelpExampleRpc("getrawmempool", "true")
        );
    bool fVerbose = false;
    if (params.size() > 0)
        fVerbose = params[0].get_bool();
    return mempoolToJSON(fVerbose);
}
UniValue getmempoolancestors(const UniValue& params, bool fHelp)