@mega65 I have just noticed a warning added in the manual
Compound instructions cannot be safely used at 1MHz or 2MHz operating modes. This is because in those operating modes it is possible for an interrupt to occur following one of the single-byte prefix instructions, but before the compound instruction executes. This will cause the processor to forget to apply the effects of the prefix to the instruction, resulting in incorrect behaviour.
Since I'm using compound instructions in the 80x50 screen editor, I have disabled SLOW/FAST commands in MEGA65 native mode. But what about 1MHz mode enforced by IEC activity? I hope the compound instructions can be still safely used in such case, otherwise I'll need to implement a workaround to make IEC routines safe in native mode.