`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?
RPCHelpMan{"preciousblock",
                "\nTreats a block as if it were received before others with the same work.\n"
                "\nA later preciousblock call can override the effect of an earlier one.\n"
                "\nThe effects of preciousblock are not retained across restarts.\n",
                {
                    {"blockhash", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "the hash of the block to mark as precious"},
                },
                RPCResults{},
                RPCExamples{
                    HelpExampleCli("preciousblock", "\"blockhash\"")
            + HelpExampleRpc("preciousblock", "\"blockhash\"")
                },
            }.ToString());
    uint256 hash(ParseHashV(request.params[0], "blockhash"));
    CBlockIndex* pblockindex;
    {
        LOCK(cs_main);
        pblockindex = LookupBlockIndex(hash);
        if (!pblockindex) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
        }
    }
    CValidationState state;
    PreciousBlock(state, Params(), pblockindex);
    if (!state.IsValid()) {
        throw JSONRPCError(RPC_DATABASE_ERROR, FormatStateMessage(state));
    }
    return NullUniValue;
}
static UniValue invalidateblock(const JSONRPCRequest& request)