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

} else if (block->nChainTx == 0) {
            
            status = "headers-only";
        } else if (block->IsValid(BLOCK_VALID_SCRIPTS)) {
            
            status = "valid-fork";
        } else if (block->IsValid(BLOCK_VALID_TREE)) {
            
            status = "valid-headers";
        } else {
            
            status = "unknown";
        }
        obj.push_back(Pair("status", status));
        res.push_back(obj);
    }
    return res;
}
UniValue mempoolInfoToJSON()
{
    UniValue ret(UniValue::VOBJ);
    ret.push_back(Pair("size", (int64_t) mempool.size()));
    ret.push_back(Pair("bytes", (int64_t) mempool.GetTotalTxSize()));
    ret.push_back(Pair("usage", (int64_t) mempool.DynamicMemoryUsage()));
    size_t maxmempool = GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000;
    ret.push_back(Pair("maxmempool", (int64_t) maxmempool));
    ret.push_back(Pair("mempoolminfee", ValueFromAmount(mempool.GetMinFee(maxmempool).GetFeePerK())));
    return ret;
}
UniValue getmempoolinfo(const UniValue& params, bool fHelp)