Syslog-ng always reads its incoming log channels to prevent your running daemons from blocking. This may result in lost messages if the output queue is full. It's therefore important to set the output queue size (termed in number of messages), which you can do globally, or on a per destination basis.
options { log_fifo_size(1000); };
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destination d_messages { file("/var/log/messages" log_fifo_size(1000); };
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Of course syslog-ng cannot widen your network bandwidth, so if your destination host lives on a noisy network, and your logtraffic extends the bandwidth of this network, syslog-ng can't do anything. It'll do its best however.