`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?
{
                                    {"desc", RPCArg::Type::STR, RPCArg::Optional::NO, "An output descriptor"},
                                    {"range", RPCArg::Type::RANGE,  "1000", "The range of HD chain indexes to explore (either end or [begin,end])"},
                                },
                            },
                        },
                        "[scanobjects,...]"},
                },
                RPCResult{
            "{\n"
            "  \"unspents\": [\n"
            "    {\n"
            "    \"txid\" : \"transactionid\",     (string) The transaction id\n"
            "    \"vout\": n,                    (numeric) the vout value\n"
            "    \"scriptPubKey\" : \"script\",    (string) the script key\n"
            "    \"desc\" : \"descriptor\",        (string) A specialized descriptor for the matched scriptPubKey\n"
            "    \"amount\" : x.xxx,             (numeric) The total amount in " + CURRENCY_UNIT + " of the unspent output\n"
            "    \"height\" : n,                 (numeric) Height of the unspent transaction output\n"
            "   }\n"
            "   ,...], \n"
            " \"total_amount\" : x.xxx,          (numeric) The total amount of all found unspent outputs in " + CURRENCY_UNIT + "\n"
            "]\n"
                },
                RPCExamples{""},
            }.ToString()
        );
    RPCTypeCheck(request.params, {UniValue::VSTR, UniValue::VARR});
    UniValue result(UniValue::VOBJ);
    if (request.params[0].get_str() == "status") {
        CoinsViewScanReserver reserver;