**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] [                                                   <=>                                                                                                                                                                  ]
```

{
    updateClusters(config, settings, config_prefix);
}
ClusterPtr Clusters::getCluster(const std::string & cluster_name) const
{
    std::lock_guard lock(mutex);
    auto it = impl.find(cluster_name);
    return (it != impl.end()) ? it->second : nullptr;
}
void Clusters::setCluster(const String & cluster_name, const std::shared_ptr<Cluster> & cluster)
{
    std::lock_guard lock(mutex);
    impl[cluster_name] = cluster;
}
void Clusters::updateClusters(const Poco::Util::AbstractConfiguration & new_config, const Settings & settings, const String & config_prefix, Poco::Util::AbstractConfiguration * old_config)
{
    Poco::Util::AbstractConfiguration::Keys new_config_keys;
    new_config.keys(config_prefix, new_config_keys);
    
    
    Poco::Util::AbstractConfiguration::Keys deleted_keys;
    if (old_config)
    {
        std::sort(new_config_keys.begin(), new_config_keys.end());
        Poco::Util::AbstractConfiguration::Keys old_config_keys;