You'd have to change the buffer of the pipe between your application and the external process, but it would slow down both apps and the operating system itself. If you want smooth output, store the data from external process instead of appending it to the text field directly and use a timer to fetch the data from your storage and feed it to the text box.

You might also want to take a look at QProcess::canReadLineStdout() and QProcess:readLineStdout() to read data line by line.