If you want to achieve that by bus, you must stick to timetable, and then you will have something like a short 2-hour stay there.
Po Lin Monastery opens 9am-6pm, and Big Buddha opens 10am-6pm. If you take Lantau bus 23 in Tung Chung departing 8:10am, you will have about an hour to see Po Lin Monastery. Then at 10am, climb the 200 steps to reach the Big Buddha.
Take the returning bus 23 to Tung Chung departing at 10:40am or 11:10am. This journey takes about 1 hour. That will be about 11:40am or 12:10pm at Tung Chung.
Then you have to connect to bus S1 to go to the airport from the bus stop not too far away. Allow 5 minutes finding time, 10 minutes waiting time and 15 minutes journey time, you will be at the airport at about 12:10pm or 12:40pm.
Of course you can take taxis, if you do, you must use blue-coloured taxis (red- or green-coloured taxis cannot go to Po Lin Monastery. Lantau Island is a big island, in fact, it is the largest island in Hong Kong, even larger than the Hong Kong Island. Allow HK$250 taxi fare, each way.
Too rush, I think.
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Philosophers say a great deal about what is absolutely necessary for science, and it is always, so far as one can see, rather naive, and probably wrong.