PA-46 pilot reported a rapid decompression during cruise which was followed by the flight crew executing a rapid descent and diverting to a precautionary landing.

Narrative

We were VFR and we picked up an IFR clearance to FL210 from ZZZ Center in order to avoid some afternoon build-ups and thunderstorms. We had 4 souls onboard and the cabin was pressurized to 8000 ft. when we were in cruise. We suffered an instant rapid depressurization in the cabin. I immediately descended during a frequency hand-off and complied with my emergency checklist. I do not know if i had deviated from my assigned altitude prior to declaring an emergency in my rapid descent below 10;000 ft. I was able to establish communication with the controller in the descent and he was very helpful. We diverted to ZZZ1 to an uneventful landing. I think there must have been something wrong with my outflow valve/overpressure valve. I am currently getting it looked at by a mechanic. My human performance was degraded and my passengers were short of breath very quickly. I was able to don my mask and run the checklist in the descent. It would have been a different story if there were solid thunderstorms under me and I will take that into consideration in the future.