`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?
}.ToString());
    int timeout = 0;
    if (!request.params[0].isNull())
        timeout = request.params[0].get_int();
    CUpdatedBlock block;
    {
        WAIT_LOCK(cs_blockchange, lock);
        block = latestblock;
        if(timeout)
            cond_blockchange.wait_for(lock, std::chrono::milliseconds(timeout), [&block]{return latestblock.height != block.height || latestblock.hash != block.hash || !IsRPCRunning(); });
        else
            cond_blockchange.wait(lock, [&block]{return latestblock.height != block.height || latestblock.hash != block.hash || !IsRPCRunning(); });
        block = latestblock;
    }
    UniValue ret(UniValue::VOBJ);
    ret.pushKV("hash", block.hash.GetHex());
    ret.pushKV("height", block.height);
    return ret;
}
static UniValue waitforblock(const JSONRPCRequest& request)
{
    if (request.fHelp || request.params.size() < 1 || request.params.size() > 2)
        throw std::runtime_error(
            RPCHelpMan{"waitforblock",
                "\nWaits for a specific new block and returns useful info about it.\n"
                "\nReturns the current block on timeout or exit.\n",
                {
                    {"blockhash", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "Block hash to wait for."},
                    {"timeout", RPCArg::Type::NUM,  "0", "Time in milliseconds to wait for a response. 0 indicates no timeout."},
                },