When running walletpassphrase on a large wallet.dat (ie about 300,000 segwit P2SH addresses) the command sometimes takes a very long time, and if command is run multiple times at once (while running slow) it causes bitcoin core to freeze (stops syncing and requires kill -9 to stop/restart)

During the locked up walletpassphrase commands being run the following messages start to appear in the debug.log:

`018-01-24 01:01:04 socket sending timeout: 1201s`

Note that this problem is intermittent, so sometimes the command runs instantly, other times it takes a few minutes, and other times it completely freezes the node.
{
    LOCK(cs_KeyStore);
    encrypted_batch = batch;
    if (!mapCryptedKeys.empty()) {
        encrypted_batch = nullptr;
        return false;
    }
    KeyMap keys_to_encrypt;
    keys_to_encrypt.swap(mapKeys); 
    for (const KeyMap::value_type& mKey : keys_to_encrypt)
    {
        const CKey &key = mKey.second;
        CPubKey vchPubKey = key.GetPubKey();
        CKeyingMaterial vchSecret(key.begin(), key.end());
        std::vector<unsigned char> vchCryptedSecret;
        if (!EncryptSecret(master_key, vchSecret, vchPubKey.GetHash(), vchCryptedSecret)) {
            encrypted_batch = nullptr;
            return false;
        }
        if (!AddCryptedKey(vchPubKey, vchCryptedSecret)) {
            encrypted_batch = nullptr;
            return false;
        }
    }
    encrypted_batch = nullptr;
    return true;
}
bool LegacyScriptPubKeyMan::GetReservedDestination(const OutputType type, bool internal, CTxDestination& address, int64_t& index, CKeyPool& keypool)