Unsolved QT Creator 4.13 hangs on file browser operations

[rustymbk1079]

Hello there.
Looking for advice on how to find out why.
Qt Creator on launching, or opening any file browser dialog (eg open project, add new file etc) hangs the computer. Mouse moves but desktop freezes - including animation of opening the browsing window.
This lasts about 3 to 6 seconds.
Same projects open and behave fine under windows and Mac Mini running same config.
I have reinstalled a couple of times including removing QT pref files.
Occasionally this also happens whilst building projects.
Any advice on how to find the problem gratefully received.
I have searched many places and cannot see anyone talking about this on recent Creator versions.

Also the case with QFileDialog when programs executed from Creator. When executed outside of creator they run smoothly.

Spec
Qt 4.13 (also was happening for me since 4.11)
SDK10.14
compiling 5.12.9 clang 64 bit
Mac OSX 10.14.6, 2.5GHz 16GB Macbook Pro