Apple's APFS is still a work in process! Fusion Drives & RAID are still not supported yet.
In fact if you have a SATA SSD you're better off staying with GUID & Mac OS Extended (Journaled) for now as APFS has issues with queuing across a SATA interface (not an issue with PCIe SSD's).
You encounter time-points when you are saving a snippet, URL link or a file and the system will pause the action, yet the process you are running won't have any problems. I've see it take almost 30 sec for the file to show up on the desktop! Yet, if you do the same thing running the same actions on a PCIe SSD it won't stall-out.