"Reading Project" very slow

I have a very large project (my top-level project contains more than 50 subdir-project, some of which are containing multiple sub-projects again).

When changing a QMake file or a QML file (sometimes, not all the time) in one of the sub-projects QtCreator gets blocked for more than 30 seconds while the "Reading Project" stage is running. Even the rotating wheel in front of the top-level project is stuck.

In older version of QtCreator I was able to work while this was going on, but the latest versions (since 14.0) blocks the whole UI, which is quite annoying, especially when working on QMake files.

Is there an option to make the processing run in the background again, like it was in older versions?

(I am running QtCreator 14.0.2, but issue can also be reproduced on 15.0.0 beta 2 or 14.0.83)

See also https://forum.qt.io/topic/159542/qtcreator-reading-project-very-slow