Click on Options and change it. If you already have a copy of Opera installed you will need to change the install path first (to convince it not to update the previous install), but then you should be able to select all users.
Because of the possibility that it's an OS/update issue, there are a couple of things to try from that angle. First, if you haven't already done it, power down the computer for a couple of minutes and restart it (just in case something in the update/reboot processes has caused a glitch)... then see if the defaults are sorted out. Next, if things are still messed up, open Windows' Control Panel and select Default Programs > "Set your default programs" > Select Opera and try making it the default program.
You were right. It is the extension "Disconnect" that is causing that problem.
I get similar speeds on my browsers that support this page. Maybe a goal could be to bring Opera into compliance if not too onerous. I guess the bigger question is: Is Opera slower? Are other people getting similar speeds on other websites with different browsers?