`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?
std::set<std::string> setDepends;
    for (const CTxIn& txin : tx.vin)
    {
        if (pool.exists(txin.prevout.hash))
            setDepends.insert(txin.prevout.hash.ToString());
    }
    UniValue depends(UniValue::VARR);
    for (const std::string& dep : setDepends)
    {
        depends.push_back(dep);
    }
    info.pushKV("depends", depends);
    UniValue spent(UniValue::VARR);
    const CTxMemPool::txiter& it = pool.mapTx.find(tx.GetHash());
    const CTxMemPool::setEntries& setChildren = pool.GetMemPoolChildren(it);
    for (CTxMemPool::txiter childiter : setChildren) {
        spent.push_back(childiter->GetTx().GetHash().ToString());
    }
    info.pushKV("spentby", spent);
    
    bool rbfStatus = false;
    RBFTransactionState rbfState = IsRBFOptIn(tx, pool);
    if (rbfState == RBFTransactionState::UNKNOWN) {
        throw JSONRPCError(RPC_MISC_ERROR, "Transaction is not in mempool");
    } else if (rbfState == RBFTransactionState::REPLACEABLE_BIP125) {
        rbfStatus = true;
    }
    info.pushKV("bip125-replaceable", rbfStatus);
}
UniValue MempoolToJSON(const CTxMemPool& pool, bool verbose)