`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?
ret_all.pushKV("minfee", (minfee == MAX_MONEY) ? 0 : minfee);
    ret_all.pushKV("minfeerate", (minfeerate == MAX_MONEY) ? 0 : minfeerate);
    ret_all.pushKV("mintxsize", mintxsize == MAX_BLOCK_SERIALIZED_SIZE ? 0 : mintxsize);
    ret_all.pushKV("outs", outputs);
    ret_all.pushKV("subsidy", GetBlockSubsidy(pindex->nHeight, Params().GetConsensus()));
    ret_all.pushKV("swtotal_size", swtotal_size);
    ret_all.pushKV("swtotal_weight", swtotal_weight);
    ret_all.pushKV("swtxs", swtxs);
    ret_all.pushKV("time", pindex->GetBlockTime());
    ret_all.pushKV("total_out", total_out);
    ret_all.pushKV("total_size", total_size);
    ret_all.pushKV("total_weight", total_weight);
    ret_all.pushKV("totalfee", totalfee);
    ret_all.pushKV("txs", (int64_t)block.vtx.size());
    ret_all.pushKV("utxo_increase", outputs - inputs);
    ret_all.pushKV("utxo_size_inc", utxo_size_inc);
    if (do_all) {
        return ret_all;
    }
    UniValue ret(UniValue::VOBJ);
    for (const std::string& stat : stats) {
        const UniValue& value = ret_all[stat];
        if (value.isNull()) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid selected statistic %s", stat));
        }
        ret.pushKV(stat, value);
    }
    return ret;
}
static UniValue savemempool(const JSONRPCRequest& request)