`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?
int64_t swtxs = 0;
    int64_t total_size = 0;
    int64_t total_weight = 0;
    int64_t utxo_size_inc = 0;
    std::vector<CAmount> fee_array;
    std::vector<std::pair<CAmount, int64_t>> feerate_array;
    std::vector<int64_t> txsize_array;
    for (const auto& tx : block.vtx) {
        outputs += tx->vout.size();
        CAmount tx_total_out = 0;
        if (loop_outputs) {
            for (const CTxOut& out : tx->vout) {
                tx_total_out += out.nValue;
                utxo_size_inc += GetSerializeSize(out, PROTOCOL_VERSION) + PER_UTXO_OVERHEAD;
            }
        }
        if (tx->IsCoinBase()) {
            continue;
        }
        inputs += tx->vin.size(); 
        total_out += tx_total_out; 
        int64_t tx_size = 0;
        if (do_calculate_size) {
            tx_size = tx->GetTotalSize();
            if (do_mediantxsize) {
                txsize_array.push_back(tx_size);
            }
            maxtxsize = std::max(maxtxsize, tx_size);
            mintxsize = std::min(mintxsize, tx_size);
            total_size += tx_size;