The my designator is used in action code in the context of composites, specifically when referring to the composite containing the note using the code, i.e. this note.
my can be used as a short form of compositeFor(this) when using with colon-operators used (only) with composites. For example, the pair of examples below have the same functional effect:
compositeFor(this):role("someRole")
my:role("someRole")
Thus the use of 'my' makes for more compact code; it has no effect on performance.
If using compositeFor() in an any other context, e.g. a composite of which the current note is not a member