I have spent 6 hours in Galleries Club South before. It can be done! Trick is to keep yourself busy.
Go for a shower, Drink lots of water and don't go overboard on the alcohol if you can help it.
CCR has great reading materials and the table service is good. You will be able to fully relax there too. The terrace I find has the comfiest seating arrangements however also has the terminal noise which some would find bothersome.
Still I would choose CCR! Or Go Lounge Hopping around T5!