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

{
    AssertLockHeld(mempool.cs);
    info.push_back(Pair("size", (int)e.GetTxSize()));
    info.push_back(Pair("fee", ValueFromAmount(e.GetFee())));
    info.push_back(Pair("modifiedfee", ValueFromAmount(e.GetModifiedFee())));
    info.push_back(Pair("time", e.GetTime()));
    info.push_back(Pair("height", (int)e.GetHeight()));
    info.push_back(Pair("startingpriority", e.GetPriority(e.GetHeight())));
    info.push_back(Pair("currentpriority", e.GetPriority(chainActive.Height())));
    info.push_back(Pair("descendantcount", e.GetCountWithDescendants()));
    info.push_back(Pair("descendantsize", e.GetSizeWithDescendants()));
    info.push_back(Pair("descendantfees", e.GetModFeesWithDescendants()));
    info.push_back(Pair("ancestorcount", e.GetCountWithAncestors()));
    info.push_back(Pair("ancestorsize", e.GetSizeWithAncestors()));
    info.push_back(Pair("ancestorfees", e.GetModFeesWithAncestors()));
    const CTransaction& tx = e.GetTx();
    set<string> setDepends;
    BOOST_FOREACH(const CTxIn& txin, tx.vin)
    {
        if (mempool.exists(txin.prevout.hash))
            setDepends.insert(txin.prevout.hash.ToString());
    }
    UniValue depends(UniValue::VARR);
    BOOST_FOREACH(const string& dep, setDepends)
    {
        depends.push_back(dep);
    }
    info.push_back(Pair("depends", depends));
}
UniValue mempoolToJSON(bool fVerbose = false)