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    Bug 
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    Resolution: Done
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    P2: Important 
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    5.5.0, 5.11.3, 5.12
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    None
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    Hardware:
 iMac - All In One PC - 5120×2880 LED IPS, Intel Quad Core i7 4.0GHz Haswell, RAM 16GB DDR3, AMD Radeon R9 M290X 2GB GDDR5, 1TB Fusion Drive
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        77a4915bf900aac0c96260018c0a4ccfbdd7c094 (qt/qtbase/5.12)
Please see the project attached. It's a simple program that repeatedly repaints a widget with single line. Then see this video: https://youtu.be/dHivEP4bJmI. It shows how the framerate of the repainting changes depending on window size on Mac OS X.
When I compared the performance on Windows 10 and Mac OS X I got these values (on the same hardware):
Widget size | Fps Mac | Fps Win10
800x600 | 350 | 622
1600x600 | 80 | 570
2560x1300 | 20 | 250
3840x2035 | n/a | 117