THX changed their spec between the SP1 and SP1.7 models. They won't let us implement THX as a toggle function anymore - it has to be a mode unto itself.
The delay is a result of a THX-required error message. If you are not using back speakers and have no need for EX processing, you can get rid of the error message by going into the 'OS' -> 'EX' menu and changing the EX control setting to 'Disable'.