I'm sorry to hear that you're having this issue.

The TV won't pass a 5.1 signal through the optical output unless that signal is passed through the TV's tuner.

However, the digital signal from the cable box to the receiver should work good, and many receivers have a delay adjustment in the event that they don't.

The lag from using both outputs simultaneously is likely because the TV is switching from a digital signal to an analog signal to play through the speakers, and this process does have a slight lag, though it shouldn't affect picture performance. The external output and the TV speakers shouldn't be used at the same time, and would naturally result in an echo because of the switch from digital to analog.

Alternatively, you might check with your cable provider to see if a reset or an update is available that might fix the issue. In some cases, resetting or refreshing the box, or firmware updates are available to help with delay issues.

Do other inputs have the same delay?