QtCharts Spectrum plot performance is sluggish

If multiple window is opened having Qt Chart spectrum performance is very bad.

Attached is example which can reproduce this.

Steps:

1. run the executable
2. click on the "QChart Create Spectrum" button to create a spectrum window.
3. Click "QChart Create Spectrum" button as many times as you can to generate Spectrum window. You will notice the sluggishness after 8 spectrum windows is created.