`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?
stats.insert(stat);
        }
    }
    const CBlock block = GetBlockChecked(pindex);
    const bool do_all = stats.size() == 0; 
    const bool do_mediantxsize = do_all || stats.count("mediantxsize") != 0;
    const bool do_medianfee = do_all || stats.count("medianfee") != 0;
    const bool do_feerate_percentiles = do_all || stats.count("feerate_percentiles") != 0;
    const bool loop_inputs = do_all || do_medianfee || do_feerate_percentiles ||
        SetHasKeys(stats, "utxo_size_inc", "totalfee", "avgfee", "avgfeerate", "minfee", "maxfee", "minfeerate", "maxfeerate");
    const bool loop_outputs = do_all || loop_inputs || stats.count("total_out");
    const bool do_calculate_size = do_mediantxsize ||
        SetHasKeys(stats, "total_size", "avgtxsize", "mintxsize", "maxtxsize", "swtotal_size");
    const bool do_calculate_weight = do_all || SetHasKeys(stats, "total_weight", "avgfeerate", "swtotal_weight", "avgfeerate", "feerate_percentiles", "minfeerate", "maxfeerate");
    const bool do_calculate_sw = do_all || SetHasKeys(stats, "swtxs", "swtotal_size", "swtotal_weight");
    if (loop_inputs && !g_txindex) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, "One or more of the selected stats requires -txindex enabled");
    }
    CAmount maxfee = 0;
    CAmount maxfeerate = 0;
    CAmount minfee = MAX_MONEY;
    CAmount minfeerate = MAX_MONEY;
    CAmount total_out = 0;
    CAmount totalfee = 0;
    int64_t inputs = 0;
    int64_t maxtxsize = 0;
    int64_t mintxsize = MAX_BLOCK_SERIALIZED_SIZE;
    int64_t outputs = 0;
    int64_t swtotal_size = 0;
    int64_t swtotal_weight = 0;