Qwt offers several optimizations how to implement real time plots - f.e in opposite to all other Qt plot packages it offers incremental painting.
You can check the oscilloscope example to see what is possible without almost no CPU usage.
Having to repaint from scratch for every incoming sample is the worst case, but even then there are strategies to speed up the repaints. But what optimzations are possible depends on your specific situation and can't be answered in general.
But if you need support for the Qwt library it doesn't make much sense to ask anywhere ales beside on the official Qwt support channels you can find on the Qwt project page !