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

"  },\n"
            "  {\n"
            "    \"height\": xxxx,\n"
            "    \"hash\": \"xxxx\",\n"
            "    \"branchlen\": 1          (numeric) length of branch connecting the tip to the main chain\n"
            "    \"status\": \"xxxx\"        (string) status of the chain (active, valid-fork, valid-headers, headers-only, invalid)\n"
            "  }\n"
            "]\n"
            "Possible values for status:\n"
            "1.  \"invalid\"               This branch contains at least one invalid block\n"
            "2.  \"headers-only\"          Not all blocks for this branch are available, but the headers are valid\n"
            "3.  \"valid-headers\"         All blocks are available for this branch, but they were never fully validated\n"
            "4.  \"valid-fork\"            This branch is not part of the active chain, but is fully validated\n"
            "5.  \"active\"                This is the tip of the active main chain, which is certainly valid\n"
            "\nExamples:\n"
            + HelpExampleCli("getchaintips", "")
            + HelpExampleRpc("getchaintips", "")
        );
    LOCK(cs_main);
    /*
     * Idea:  the set of chain tips is chainActive.tip, plus orphan blocks which do not have another orphan building off of them. 
     * Algorithm:
     *  - Make one pass through mapBlockIndex, picking out the orphan blocks, and also storing a set of the orphan block's pprev pointers.
     *  - Iterate through the orphan blocks. If the block isn't pointed to by another orphan, it is a chain tip.
     *  - add chainActive.Tip()
     */
    std::set<const CBlockIndex*, CompareBlocksByHeight> setTips;
    std::set<const CBlockIndex*> setOrphans;
    std::set<const CBlockIndex*> setPrevs;
    BOOST_FOREACH(const PAIRTYPE(const uint256, CBlockIndex*)& item, mapBlockIndex)