I just joined Optus ADSL+ Broadband a few weeks ago, after being with Telstra for decades. I was told by the Optus Sales Team that my Broadband would stay the same speed once I crossed over.
And for three weeks it was working the same at roughly 6.5Mbps each day, all day.
This afternoon the internet speed suddenly dropped to 2.6Mbps. I can't watch any videos, they keep buffering. It takes 3-5 minutes to load a page like Facebook, scrolling is impossible.
I turned off my laptop, turn off my modem, restarted everything. Same.
I called Optus Customer Care who told me that 2.6Mbps was good for my distance from the Exchange Node. I kept telling her that I was previously getting 6.5Mbps, but she wouldn't listen to me.
Even at the slower speed pages should load properly. I suspect there is an underlying issue and I would suggest hitting up support about a connection problem rather than arguing about speeds. At least that way they can't fob you off.
I don't know what operating system you are using, but Networx should be sitting around on the bottom right. Right clicking on the icon and selecting "Show Graph" should get you a display of your downloading speed over time. You can go into settings and put "minute ticks" on the graph display.
Then download something big like a Linux distro on Aarnet mirror
Or a Windows service pack
Watch the graph for whether the download is constant or all broken up by packet loss. If it goes up and down and all over the place, but is pretty solid, that is congestion. If it's solid and flat lined, that is ASDL line speed limit. If it stops and starts, and looks miserable, then that is bad juju.