I'm using Ubuntu 16.04, QT 5.5.1. Both Musescore and Polyphone open a "Loading..." dialog for no reason shortly after application startup.
I found the Musescore bug entry, which listed a workaround commit.
github.com/musescore/MuseScore/commit/89...1981c5dc530d349ffd14
Reading the comments of QTBUG-47042, each QProgressDialog object will automatically display after being constructed (
bugreports.qt.io/browse/QTBUG-47042?focu...panel#comment-301657). It was intended to be created and destroyed once per operation. You need to either create it during operations, or call ->reset() after creating.
Interestingly enough, loading/saving big soundfonts on Windows 1.7 take a long time, but don't produce a dialog...