You didn't say where you are located. Assuming that it is still warm enough where you are, I would start by getting the infested plant out of my house, especially if you have any other house plants. Once outside it is easier to work with. Large (1 cm = abt 1/2 inch) and slow moving sounds like you could hand pick them off or shake them off into a bowl of soapy water. If your tree is manageable size, I would think about wrapping the pot surface so the soil doesn't fall out and then dunking the whole tree into a trash bin full of soapy water. Otherwise just keep hosing it off with hard sprays of water, including all the undersides of leaves.
Are you seeing any chewed leaves or damage other than just looking sickly?
You are right about the non-typical, doesn't sound like any of the common things that afflict hibiscus. I hope someone else comes along who can be more help re what your bug is.
There is something called a hibiscus beetle:
It is black but only about 3 mm long and spends most of its time in hibiscus flowers.