The `getaddressesbylabel` API spends a lot more time than the `getaddressesbyaccount` API.

I generated addresses by the `getnewaddress` API, then, I measured time that the `getaddressesby*` API spends. It seems the algorithm of the `getaddressesbylabel` API takes **O(N ^ 2)** time! The results of my survey are as follows.

### getaddressesbylabel

bitcoind v0.17.1 on Ubuntu 16.04 (AWS EC2 t3.large)

#### 11,001 addresses

```sh
time sudo /usr/local/bin/bitcoin-cli -conf=/etc/bitcoin/bitcoin.conf getaddressesbylabel ""
# (omitted)
real    0m1.230s # 1.2s
user    0m0.044s
sys 0m0.048s
```

#### 111,001 addresses

```sh
time sudo /usr/local/bin/bitcoin-cli -conf=/etc/bitcoin/bitcoin.conf getaddressesbylabel ""
# (omitted)
real    1m37.027s # 97.0s
user    0m0.408s
sys 0m0.544s
```

#### 211,001 addresses

```sh
time sudo /usr/local/bin/bitcoin-cli -conf=/etc/bitcoin/bitcoin.conf getaddressesbylabel ""
# (omitted)
real    6m3.727s # 363.7s
user    0m0.800s
sys 0m0.900s
```

### getaddressesbyaccount

bitcoind v0.16.3 on Ubuntu 16.04 (AWS EC2 t3.large)

#### 11,001 addresses

```sh
time sudo /usr/local/bin/bitcoin-cli -conf=/etc/bitcoin/bitcoin.conf getaddressesbyaccount ""
# (omitted)
real    0m0.500s # 0.5s
user    0m0.092s
sys 0m0.160s
```

#### 111,001 addresses

```sh
time sudo /usr/local/bin/bitcoin-cli -conf=/etc/bitcoin/bitcoin.conf getaddressesbyaccount ""
# (omitted)
real    0m1.361s # 1.4s
user    0m0.168s
sys 0m0.348s
```

#### 211,001 addresses

```sh
time sudo /usr/local/bin/bitcoin-cli -conf=/etc/bitcoin/bitcoin.conf getaddressesbyaccount ""
# (omitted)
real    0m2.220s # 2.2s
user    0m0.384s
sys 0m0.532s
```

pwallet->BlockUntilSyncedToCurrentChain();
    LockAnnotation lock(::cs_main); 
    auto locked_chain = pwallet->chain().lock();
    LOCK(pwallet->cs_wallet);
    
    CTxDestination dest = DecodeDestination(request.params[0].get_str());
    if (!IsValidDestination(dest)) {
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid Bitcoin address");
    }
    CScript scriptPubKey = GetScriptForDestination(dest);
    if (!IsMine(*pwallet, scriptPubKey)) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Address not found in wallet");
    }
    
    int nMinDepth = 1;
    if (!request.params[1].isNull())
        nMinDepth = request.params[1].get_int();
    
    CAmount nAmount = 0;
    for (const std::pair<const uint256, CWalletTx>& pairWtx : pwallet->mapWallet) {
        const CWalletTx& wtx = pairWtx.second;
        if (wtx.IsCoinBase() || !CheckFinalTx(*wtx.tx))
            continue;
        for (const CTxOut& txout : wtx.tx->vout)
            if (txout.scriptPubKey == scriptPubKey)
                if (wtx.GetDepthInMainChain(*locked_chain) >= nMinDepth)
                    nAmount += txout.nValue;
    }
    return  ValueFromAmount(nAmount);
}