`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?
int nTxDiff = pindex->nChainTx - pindexPast->nChainTx;
    UniValue ret(UniValue::VOBJ);
    ret.pushKV("time", (int64_t)pindex->nTime);
    ret.pushKV("txcount", (int64_t)pindex->nChainTx);
    ret.pushKV("window_final_block_hash", pindex->GetBlockHash().GetHex());
    ret.pushKV("window_block_count", blockcount);
    if (blockcount > 0) {
        ret.pushKV("window_tx_count", nTxDiff);
        ret.pushKV("window_interval", nTimeDiff);
        if (nTimeDiff > 0) {
            ret.pushKV("txrate", ((double)nTxDiff) / nTimeDiff);
        }
    }
    return ret;
}
template<typename T>
static T CalculateTruncatedMedian(std::vector<T>& scores)
{
    size_t size = scores.size();
    if (size == 0) {
        return 0;
    }
    std::sort(scores.begin(), scores.end());
    if (size % 2 == 0) {
        return (scores[size / 2 - 1] + scores[size / 2]) / 2;
    } else {
        return scores[size / 2];
    }
}
void CalculatePercentilesByWeight(CAmount result[NUM_GETBLOCKSTATS_PERCENTILES], std::vector<std::pair<CAmount, int64_t>>& scores, int64_t total_weight)