The scene is where the astronauts are talking to ground control about HAL's erratic behavior.
The astronauts are somewhere near Jupiter and ground control is on Earth so the minimum communication transit time is 35 minutes. If they were true to science, the communication would near to impossible, so did Kubrick cheat and use instantaneous communication?
No cheating. They are not interacting just receiving messages.
Edit 1 Another recorded message. Birthday greetings for Frank. (And thanks to the youtube uploaders.)
Edit 2 The news interview Keith Morrison describes in his comment to this answer - which he points out the BBC reporter mentions the time lag.
Edit 3 regarding real deep space communication. The Voyager probes are outside of the solar system now and are sending a 23 watt signal. A cellphone uses 3 watts. So I think a manned spaceship would certainly put power into the communications. Hopefully the AI doesn't sabotage it. See https://science.howstuffworks.com/question431.htm
In the book it is very specific that there is an ever increasing time delay between Earth and the Discovery, it is used as a plot device to emphasise the isolation that the crew feel. As the book goes on and Bowman is left as the sole survivor he muses on the fact that not only will no help be coming but that he will be long dead before mission control even knows it.
It's made very clear in both the book and the movie that there is a long time lag for radio communications with Discovery. For example, in the birthday scene in the movie, the parents' transmission is one way and refers to Frank Poole's previous transmission. This all makes sense, because Clarke was writing hard SF with as much scientific accuracy as possible. Clarke himself was one of the originators of the idea of a communications satellite, so it's not as though he'd be ignorant of things like the fact that radio signals propagate at c.
However, later in the movie, after Dave Bowman passes through the star gate, it's equally clear that he is zooming around faster than the speed of light. The implication is that this is something like a wormhole, which is perfectly consistent with relativity. (The speed limit c is a local one.) IIRC the book is vague and hand-wavy about this sort of thing, but mentions something like an idea that the 1:4:9 ratio of the spatial dimensions of the monolith is actually an infinite sequence extending into infinitely many dimensions. Basically this is an example of Clarke's laws.
The "3000" sequel story also makes clear that the monoliths are capable to send and receive messages over a distance of 500 light years, but only at the speed of light. The result is significant action in the year 3,000.
