Optus adsl2 is slow

That modem has a lot of problems which Optus are aware of. Call them and get a refund which is what I did... buy a billion.

Unfortunately most problems like this are caused by poor telephone line quality which makes it a BT problem. See if your neighbours are getting faster ADSL speeds. IF they are then report an ADSL fault to BT.
Also try disconnecting your house wiring right back to the master socket and test your ADSL from there.
Remember that the ADSL connection is only between your computer and your local BT sub-exchange from there to the rest of the internet is a high speed pipe shared between all the ISP's used in your area, the various ISP's just rent a portion of the bandwith of the pipe.