Qt application performance gets low after the display is rotated left.
root@apalis-imx6:~# xrandr -o left
In normal display, the application draws line. And /usr/bin/X consumes 23.3% of CUP resource. The Qt5.5 application uses around 74% of CUP. After the display is rotated. /usr/bin/X consumes up to 65% of CUP resource. And the Qt application performance also gets lower.
The test application is uploaded at https://share.toradex.com/jc5c59c0aoxfjok
It seems X windows limits the performance.