Don't get me wrong but are you aware that you are comparing a Dual Core 2GHz (so 2 Core at 2GHz) with 4GB of RAM and a low power ARM processor at 700MHz with 512MB or RAM ? Not to mention the GPU power levels that are also pretty different.
The first thing I'd check is how much data you are trying to process and show and optimize that.
I have solved it. I discovered that I needed to refresh my plot, which is to say I needed to re-plot my plot. Let me:
//I am just skipping to the click function of my button
QCPItemStraightLine* line = (QCPItemStraightLine) (ui->widget->item());
ui->widget->replot(); //This line solved the problem.