Handling Efficiency issues

Hi all for some reason i am doing something like this
for i in Message.objects.all():
i.read_at = datetime.now()
i.save()

now the number of Message instances are likely to increase by a lot
with time . My question is is it likely that this code piece will
become very slow over time. If yes how can handle this more
efficiently

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A-M-I-T S|S

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