`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?
!index->LookupFilterHeader(block_index, filter_header)) {
        int err_code;
        std::string errmsg = "Filter not found.";
        if (!block_was_connected) {
            err_code = RPC_INVALID_ADDRESS_OR_KEY;
            errmsg += " Block was not connected to active chain.";
        } else if (!index_ready) {
            err_code = RPC_MISC_ERROR;
            errmsg += " Block filters are still in the process of being indexed.";
        } else {
            err_code = RPC_INTERNAL_ERROR;
            errmsg += " This error is unexpected and indicates index corruption.";
        }
        throw JSONRPCError(err_code, errmsg);
    }
    UniValue ret(UniValue::VOBJ);
    ret.pushKV("filter", HexStr(filter.GetEncodedFilter()));
    ret.pushKV("header", filter_header.GetHex());
    return ret;
}
static const CRPCCommand commands[] =
{ 
  
    { "blockchain",         "getblockchaininfo",      &getblockchaininfo,      {} },
    { "blockchain",         "getchaintxstats",        &getchaintxstats,        {"nblocks", "blockhash"} },
    { "blockchain",         "getblockstats",          &getblockstats,          {"hash_or_height", "stats"} },
    { "blockchain",         "getbestblockhash",       &getbestblockhash,       {} },
    { "blockchain",         "getblockcount",          &getblockcount,          {} },
    { "blockchain",         "getblock",               &getblock,               {"blockhash","verbosity|verbose"} },
    { "blockchain",         "getblockhash",           &getblockhash,           {"height"} },