I have the same problem but I’m not sure if this is a basic firefly problem. My slugs show similar symtoms when initiating heavy I/O load like cp -R /dir1 /dir2. I experience this behavior since the first versions of UNLSUNG.
The work around for me for firefly was to deactivate the directory ‘Rescan Interval’. Now the slug doesn’t freeze during playback and I start scaning manually 🙁 – but it works.
Maybe someone else can explain these slug-experience?