**Describe the situation**
```
$ ./clickhouse-21.3.3 client --query "SELECT * FROM system.numbers_mt FORMAT Native" | pv > /dev/null
^C,1GiB 0:00:45 [ 710MiB/s] [                                                                                                                                                                                               <=>                      ]

$ ./clickhouse-21.3.3 client --compress 0 --query "SELECT * FROM system.numbers_mt FORMAT Native" | pv > /dev/null
^C,2GiB 0:00:12 [1,48GiB/s] [                                                   <=>                                                                                                                                                                  ]
```

old_config->keys(config_prefix, old_config_keys);
        std::sort(old_config_keys.begin(), old_config_keys.end());
        std::set_difference(
            old_config_keys.begin(), old_config_keys.end(), new_config_keys.begin(), new_config_keys.end(), std::back_inserter(deleted_keys));
    }
    std::lock_guard lock(mutex);
    
    if (old_config)
    {
        for (const auto & key : deleted_keys)
            impl.erase(key);
    }
    else
        impl.clear();
    for (const auto & key : new_config_keys)
    {
        if (key.find('.') != String::npos)
            throw Exception("Cluster names with dots are not supported: '" + key + "'", ErrorCodes::SYNTAX_ERROR);
        
        if (!old_config || !isSameConfiguration(new_config, *old_config, config_prefix + "." + key))
            impl[key] = std::make_shared<Cluster>(new_config, settings, config_prefix, key);
    }
}
Clusters::Impl Clusters::getContainer() const
{
    std::lock_guard lock(mutex);
    
    return impl;
}
Cluster::Cluster(const Poco::Util::AbstractConfiguration & config,