First things first: it worked! Thanks a lot
Second: a small change in apache's configuration is needed: I erroneously configured my module as a Fixup handler, which caused it to be called a huge number of times per line, that amplified the problem. Configuring it as a log handler correctly writes one line at a time. Anyway, I also tested it with the wrong apache configuration, and it still worked, so your solution is absolutely right.
Under heavy load, what you're seeing is one process being put into a wait state before the newline is output. You don't mention it, but did your logfile also contain blank lines? It should've.
Right. I didn't mention it since I pasted an example that contained a blank line. Nevertheless, it was not completely evident. My fault.
In theory, there is no difference between theory and practice. In practice, there is.