slow rendering on high resolution no machine 7 desktop

We are facing a slow rendering issue on high resolution displays (4000x2000 or higher) in a QT 5.12.9 Application running on nomachine 7 windows client connected to a nomachine 7 linux server.  We are running the application on Redhat 7.6 machine.

Please note that we are facing this slow rendering issue only on nomachine 7 setup and not the old no machine 5 setup that we had earlier.

Also with QT 5.2.0 and QT 4.8.6 the issue is severe.

With Qt5.12.9 rendering is slow but not as severe as Qt 5.2.0 and QT 4.8.6 as mentioned above.

With QT 4.7.5 and below the rendering works fine.

To reproduce - We used a nomachine 7 remote desktop with high resolution (4000x2000 or higher) and drew some QPixmap(black screen) using a QPainter and we could see the black screen drawing very slow from top to bottom on Qt 5.2.0. with QT 5.12.9 it was slow but not as severe as QT 5.2.0.

Other example we used was drawing lines on mouse movement and we could also see the lines drawing very slow once the mouse was moved from one position to another especially on QT 5.2.0 and QT 4.8.6.