Listing subgroups of given index

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I need to consider subgroups of a given index to a group. So far I used the .conjugacy_classes_subgroups() method but this takes too much time (and memory), since it computes all subgroups. For example I used:

G = PrimitiveGroup(81,146);
ConjClassList = G..conjugacy_classes_subgroups()

then the computer runs out of memory. But there might be only a few subgroups with a given index. Is there a relatviely efficient way to do so?