Loading large CMake projects is slow when using Ninja generator

When using a large CMake project with the Ninja generator, there are long delays when any of the following occurs:

• The Finish button on the Run CMake window is pressed,
• The build configuration is switched, or
• The session is loaded
  The delays are not noticeable when using the Makefile generator instead of Ninja.

Steps to reproduce:

1. Decompress attached slow_loading_project.tar.gz
2. Open the CMakeLists.txt inside it with Qt Creator
3. On the Run CMake window, select the Ninja generator
4. Press Run CMake
5. Press Finish
6. Try resizing the window or something that should cause the UI to change immediately.

The attached synthetic project will cause a delay of few seconds during which Qt Creator's UI is unresponsive and its CPU usage is high. I have seen a real world project that causes a delay of about 10 minutes.

I've attached an interactive SVG flamegraph showing which functions were using the CPU time during the delay.