Performance issue with NSFullSizeContentViewWindowMask flag

Brief story:

Usage of the following code snippet leads to severe performance losses in Qt applications.

window.styleMask |= NSFullSizeContentViewWindowMask;

Long story:

I was trying to develop better-looking macOS apps, hence I decided to use native Objective-C codes to get rid of the titlebar of my application with full size content view like below:

I'm using following method to achive my goal:

auto mainWindow = new QWindow;
auto nsView = (NSView*) mainWindow->winId();
auto window = [nsView window];
[window setTitlebarAppearsTransparent: YES];
[window setTitleVisibility: NSWindowTitleHidden];
[window setAppearance: [NSAppearance appearanceNamed:NSAppearanceNameAqua]];
window.styleMask |= NSFullSizeContentViewWindowMask; //Problematic line

Usage of the last line of the code is affecting the general application painting performance. I did a performance compare with a very simple test application (which I've attached below). I have several AnimatedImage objects in my simple qml application to inspect general rendering performance of Qt applications with and without usage of above problematic code.

Test results

Before:

After:

You can see the CPU usage of the "macforce" app. Cpu load changes from %2.6 to %50.7 with the usage of a single line in the code.