> A key requirement for logging is to write statements, from any thread, in order, to a single log-file.

I do not agree that it's a necessary requirement that all threads must write to a single log-file.

> Formatting strings, required by a log system, is a slow operation.

I also do not agree that it's a necessary requirement that a log system must format strings. Binary log files have their uses. I've used Google Protocol Buffers quite happily. They may not be appropriate for extremely high-speed logs, like a low-latency trading system implies, but they have their uses. I'd be tempted to try something like Cap'n Proto, if I were taking a whack at it.