PFS does not immediately write to disk.. but it delays writes.
This delay can be many seconds.
Because of this if your machine crashes or you turn the computer off, it could be that writes were not finished.
As you know Amiga Demos and Games very often disable the OS off.
This means if you run games or demos its could happen that the game /demo disables the OS and any not yet written delayed write gets lost.
Regarding IDE speed.
Not all devices support maximum speed.
You should start with FASTSPEED=1 and see if this runs stable for you. If it does then can increase to FASTSPEED=2 and see if this runs stable for, and so on. Find this way out which speed is the maximum stable for your system.