We have experienced something similar to this on our Samsung TVs. After turning on the TV, it can be very slow when trying to change settings or inputs. This is a result of the software and the processor in this model of the TV, not your individual TV. It may have been improved over your previous TV due to a software update which generally are to fix bugs or stability issues. The smart menu processor is independent of the video processing and so it will not affect the input lag or picture quality.

We are not able to test the sound occurrence at the moment, but this may be due to the TV or the cable box. If you can test with another TV you can see if it is because of the TV or the box. As you said, this may be a feature to transition between channels. Check your cable box settings to see if it can be disabled.