CX251 scheduled departure delayed to 12:25.
FR24 shows recent CX251 has delayed departures of 30-60 mins and arrivals from on time to 15-30mins delay;
whereas recent CX255 has delayed departures of 45-75 mins and delayed arrivals 10-40mins.
In view of the statistics unless delayed further you will arrive fairly on time. The hold is probably to avoid the plane arriving too early (due to schedule padding).

Baggage handling isn't very efficient in HK Airport AFAIK because of general staffing shortage. There really are a lot of bags underneath. Hope it'd turn out better for you later on.

Have they told you just the bag to retrieve or uncertain about flight loading for the standby request?