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(
            "invalidateblock \"hash\"\n"
            "\nPermanently marks a block as invalid, as if it violated a consensus rule.\n"
            "\nArguments:\n"
            "1. hash   (string, required) the hash of the block to mark as invalid\n"
            "\nResult:\n"
            "\nExamples:\n"
            + HelpExampleCli("invalidateblock", "\"blockhash\"")
            + HelpExampleRpc("invalidateblock", "\"blockhash\"")
        );
    std::string strHash = params[0].get_str();
    uint256 hash(uint256S(strHash));
    CValidationState state;
    {
        LOCK(cs_main);
        if (mapBlockIndex.count(hash) == 0)
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
        CBlockIndex* pblockindex = mapBlockIndex[hash];
        InvalidateBlock(state, Params(), pblockindex);
    }
    if (state.IsValid()) {
        ActivateBestChain(state, Params(), NULL, g_connman.get());
    }
    if (!state.IsValid()) {
        throw JSONRPCError(RPC_DATABASE_ERROR, state.GetRejectReason());