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
```

}
    if (!request.params[6].isNull()) {
        coin_control.m_confirm_target = ParseConfirmTarget(request.params[6]);
    }
    if (!request.params[7].isNull()) {
        if (!FeeModeFromString(request.params[7].get_str(), coin_control.m_fee_mode)) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid estimate_mode parameter");
        }
    }
    std::set<CTxDestination> destinations;
    std::vector<CRecipient> vecSend;
    CAmount totalAmount = 0;
    std::vector<std::string> keys = sendTo.getKeys();
    for (const std::string& name_ : keys) {
        CTxDestination dest = DecodeDestination(name_);
        if (!IsValidDestination(dest)) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, std::string("Invalid Bitcoin address: ") + name_);
        }
        if (destinations.count(dest)) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, std::string("Invalid parameter, duplicated address: ") + name_);
        }
        destinations.insert(dest);
        CScript scriptPubKey = GetScriptForDestination(dest);
        CAmount nAmount = AmountFromValue(sendTo[name_]);
        if (nAmount <= 0)
            throw JSONRPCError(RPC_TYPE_ERROR, "Invalid amount for send");
        totalAmount += nAmount;
        bool fSubtractFeeFromAmount = false;
        for (unsigned int idx = 0; idx < subtractFeeFromAmount.size(); idx++) {
            const UniValue& addr = subtractFeeFromAmount[idx];