`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?
const CBlockIndex* block = tip;
        assert(block);
        while (block->pprev && (block->pprev->nStatus & BLOCK_HAVE_DATA)) {
            block = block->pprev;
        }
        obj.pushKV("pruneheight",        block->nHeight);
        
        bool automatic_pruning = (gArgs.GetArg("-prune", 0) != 1);
        obj.pushKV("automatic_pruning",  automatic_pruning);
        if (automatic_pruning) {
            obj.pushKV("prune_target_size",  nPruneTarget);
        }
    }
    const Consensus::Params& consensusParams = Params().GetConsensus();
    UniValue softforks(UniValue::VARR);
    UniValue bip9_softforks(UniValue::VOBJ);
    softforks.push_back(SoftForkDesc("bip34", 2, tip, consensusParams));
    softforks.push_back(SoftForkDesc("bip66", 3, tip, consensusParams));
    softforks.push_back(SoftForkDesc("bip65", 4, tip, consensusParams));
    for (int pos = Consensus::DEPLOYMENT_CSV; pos != Consensus::MAX_VERSION_BITS_DEPLOYMENTS; ++pos) {
        BIP9SoftForkDescPushBack(bip9_softforks, consensusParams, static_cast<Consensus::DeploymentPos>(pos));
    }
    obj.pushKV("softforks",             softforks);
    obj.pushKV("bip9_softforks", bip9_softforks);
    obj.pushKV("warnings", GetWarnings("statusbar"));
    return obj;
}
struct CompareBlocksByHeight
{
    bool operator()(const CBlockIndex* a, const CBlockIndex* b) const