only reason for using sysread/syswrite with select is because they don't wait for complete input.

No, primary reason to use sysread/syswrite with select, is because readline and print are not working with select.

And I reason to use select in application, is that I have several processes which send message, and I am not sure which of them will send next message and when, so I need to select between their pipes.

And if your messages are not some exact multiple of 4K, the last line of every 4k block will be a partial message, and your wrapper will therefore block until the next 4k block has been filled and passed through, before that message will be completed

That's true for blocking pipes, anyway. Post is about blocking pipes. Yes, with blocking pipes master process performance would be limiting factor for all processes. I didn't see when it's a problem in my IPC applications - either master is fast, or messages are rare/small, or child need a reply anyway, just after sending a message.