You seem have contradictory requirements. You want it to work with multiple threads. But you also want buffered output. But buffered output means that one thread will write out a block, ending with part of a line, then other threads will write out data before the first thread finishes its line. That will result in corrupted output.

As others have said, try it and see what performance looks like. My bet is that performance will be just fine. After all flushing after a print just means that a block of data is sent to the filesystem. It does not mean that a write happens immediately to disk. Sure, one will happen when the filesystem gets around to it. But pages are generally left dirty in memory for a bit. And as I noted above, the flushes are necessary anyways if you want to be able to avoid having multiple threads cause you headaches.