`getblock` is slow on verbosity, it's happened because `LOCK(cs_main)` applied for whole function including JSON generation
https://github.com/bitcoin/bitcoin/blob/ef70f9b52b851c7997a9f1a0834714e3eebc1fd8/src/rpc/blockchain.cpp#L810

I added few lines which print spent time:
```cpp
    LOCK(cs_main);
    auto t1 = std::chrono::high_resolution_clock::now();
...
    auto t2 = std::chrono::high_resolution_clock::now();
    auto ret = blockToJSON(block, pblockindex, verbosity >= 2);
    auto t3 = std::chrono::high_resolution_clock::now();
    std::cout << "blockToJSON " << pblockindex->nHeight << ": " << std::chrono::duration_cast<std::chrono::microseconds>(t2 - t1).count() << " " << std::chrono::duration_cast<std::chrono::microseconds>(t3 - t1).count() << std::endl;
    return ret;
```

and numbers looks like:
```
blockToJSON 570019: 14727 201870
blockToJSON 570022: 16049 225224
blockToJSON 570021: 33776 217398
blockToJSON 570020: 21754 191669
blockToJSON 570023: 30949 236941
blockToJSON 570025: 23346 223673
blockToJSON 570024: 18732 203002
blockToJSON 570026: 17467 216213
blockToJSON 570027: 16658 217395
blockToJSON 570028: 19015 219183
```

i.e. everything before JSON generation takes 15-30ms, while JSON takes minimum 190ms and lock `cs_main` for all this time. As result it's possible extract only ~5 blocks per second.

Looking on the code I think it's possible copy data from `CBlockIndex` to extra structure, so in this case `cs_main` can be unlocked right after `GetBlockChecked`.
https://github.com/bitcoin/bitcoin/blob/ef70f9b52b851c7997a9f1a0834714e3eebc1fd8/src/rpc/blockchain.cpp#L828
As result `getblock` in verbosity mode should be much faster (in case if `-rpcthreads` will be adjusted too).

Does this make sense?
"  \"bestblock\":  \"hash\",    (string) The hash of the block at the tip of the chain\n"
            "  \"confirmations\" : n,       (numeric) The number of confirmations\n"
            "  \"value\" : x.xxx,           (numeric) The transaction value in " + CURRENCY_UNIT + "\n"
            "  \"scriptPubKey\" : {         (json object)\n"
            "     \"asm\" : \"code\",       (string) \n"
            "     \"hex\" : \"hex\",        (string) \n"
            "     \"reqSigs\" : n,          (numeric) Number of required signatures\n"
            "     \"type\" : \"pubkeyhash\", (string) The type, eg pubkeyhash\n"
            "     \"addresses\" : [          (array of string) array of bitcoin addresses\n"
            "        \"address\"     (string) bitcoin address\n"
            "        ,...\n"
            "     ]\n"
            "  },\n"
            "  \"coinbase\" : true|false   (boolean) Coinbase or not\n"
            "}\n"
                },
                RPCExamples{
            "\nGet unspent transactions\n"
            + HelpExampleCli("listunspent", "") +
            "\nView the details\n"
            + HelpExampleCli("gettxout", "\"txid\" 1") +
            "\nAs a JSON-RPC call\n"
            + HelpExampleRpc("gettxout", "\"txid\", 1")
                },
            }.ToString());
    LOCK(cs_main);
    UniValue ret(UniValue::VOBJ);
    uint256 hash(ParseHashV(request.params[0], "txid"));
    int n = request.params[1].get_int();
    COutPoint out(hash, n);