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

"  \"verificationprogress\": xxxx, (numeric) estimate of verification progress [0..1]\n"
            "  \"chainwork\": \"xxxx\"     (string) total amount of work in active chain, in hexadecimal\n"
            "  \"pruned\": xx,             (boolean) if the blocks are subject to pruning\n"
            "  \"pruneheight\": xxxxxx,    (numeric) heighest block available\n"
            "  \"softforks\": [            (array) status of softforks in progress\n"
            "     {\n"
            "        \"id\": \"xxxx\",        (string) name of softfork\n"
            "        \"version\": xx,         (numeric) block version\n"
            "        \"reject\": {            (object) progress toward rejecting pre-softfork blocks\n"
            "           \"status\": xx,       (boolean) true if threshold reached\n"
            "        },\n"
            "     }, ...\n"
            "  ],\n"
            "  \"bip9_softforks\": {          (object) status of BIP9 softforks in progress\n"
            "     \"xxxx\" : {                (string) name of the softfork\n"
            "        \"status\": \"xxxx\",    (string) one of \"defined\", \"started\", \"locked_in\", \"active\", \"failed\"\n"
            "        \"bit\": xx,             (numeric) the bit (0-28) in the block version field used to signal this softfork (only for \"started\" status)\n"
            "        \"startTime\": xx,       (numeric) the minimum median time past of a block at which the bit gains its meaning\n"
            "        \"timeout\": xx          (numeric) the median time past of a block at which the deployment is considered failed if not yet locked in\n"
            "     }\n"
            "  }\n"
            "}\n"
            "\nExamples:\n"
            + HelpExampleCli("getblockchaininfo", "")
            + HelpExampleRpc("getblockchaininfo", "")
        );
    LOCK(cs_main);
    UniValue obj(UniValue::VOBJ);
    obj.push_back(Pair("chain",                 Params().NetworkIDString()));
    obj.push_back(Pair("blocks",                (int)chainActive.Height()));