`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?
{
    
    UniValue result(UniValue::VOBJ);
    result.pushKV("hash", blockindex->GetBlockHash().GetHex());
    const CBlockIndex* pnext;
    int confirmations = ComputeNextBlockAndDepth(tip, blockindex, pnext);
    result.pushKV("confirmations", confirmations);
    result.pushKV("strippedsize", (int)::GetSerializeSize(block, PROTOCOL_VERSION | SERIALIZE_TRANSACTION_NO_WITNESS));
    result.pushKV("size", (int)::GetSerializeSize(block, PROTOCOL_VERSION));
    result.pushKV("weight", (int)::GetBlockWeight(block));
    result.pushKV("height", blockindex->nHeight);
    result.pushKV("version", block.nVersion);
    result.pushKV("versionHex", strprintf("%08x", block.nVersion));
    result.pushKV("merkleroot", block.hashMerkleRoot.GetHex());
    UniValue txs(UniValue::VARR);
    for(const auto& tx : block.vtx)
    {
        if(txDetails)
        {
            UniValue objTx(UniValue::VOBJ);
            TxToUniv(*tx, uint256(), objTx, true, RPCSerializationFlags());
            txs.push_back(objTx);
        }
        else
            txs.push_back(tx->GetHash().GetHex());
    }
    result.pushKV("tx", txs);
    result.pushKV("time", block.GetBlockTime());
    result.pushKV("mediantime", (int64_t)blockindex->GetMedianTimePast());