PVRTexTool infinite compressing Unity 5.4.0f3 freezes, cpu 100%

Today I installed Unity 5.4 and for some reason I can't manage to compress texture anymore. I'm on a 6 year old PC with a CPU Q9550, Gfx card AMD R9, 6GB RAM. But when I select a single texture to compress it I see 4x PVRTexTool in my taskmanager and then I can sit and wait for 12 hours without any response. The CPU is at 100% all the time and this doesn't stop unless I kill Unity by hand.

Unity has become unusable since 5.4. What happened that my system can't handle this texture compressing?

Here a screenshot if no believe



Edit:

It actually took me 15 minutes to compress 1 texture. Within Unity 5.3.4 it took me not even 5 seconds.