This problem has started up on my HS APFS install as well. At first there were zero issues. Over the last week, even though the reported free space has remained more or less constant, the file system performance as gone into the toilet. Today it was so bad that I decided to carefully close ALL applications and then do a normal shutdown. Everything went okay until well into the shutdown, when after the expected black screen just before powerdown, I got a pinkish screen with vertical dashed lines running every inch or so and the system HUNG! I gave it 10 minutes or so and finally did a forced powerdown. The system rebooted and I found a LOT of problems. All my message threads were gone (so much for messages in the cloud!), all my Safari extensions were gone, mail had to rebuild the index and I wound up with about 500 messages in recovered folders so I had to sort thru that mess. I don't know what else is screwed up this point.
This seems is a bit late in the development cycle for this level of issues to be popping up, especially in the file system. Given that the APFS rollout in watchOS and iOS gave no issues, I certainly anticipated that APFS in macOS would be at least solid even if slower during the beta. Oh well, I knew the risk with Apple macOS betas. Seeing the issues that APFS is encountering in beta does NOT bode well for getting it solid in the few months remaining before GM. I have a week old clone of my Sierra environment and I may well be taking the steps to cleanse my system of APFS and returning to the know world of HFS+!
UPDATE!
I have had enough! Doing a CC clone to a local HFS+ volume of the HS environment. I will then boot to recovery, delete the AFPS container and install a fresh copy of Sierra followed by a migration of the CC clone.
I sincerely HOPE that Apple can pulll APFS out of the *******, but at this late stage, I have my doubts that it will be ready for the fall GM, at least for the masses. I think Apple really needs to think hard if they want the nightmare of rolling APFS out to the masses before it is rock solid and at least as fast and robust as HFS+!