I don't like this idea for two reasons.
For one, I think this is a viable strategy: you are a detective and have a tazer, you can tell someone to freeze and then you can go up to that someone and look at them for a couple seconds. If that player is a T, they often start attacking because they think they are about to get tazed. This strategy facilitates a fast-paced round, helps prevent delaying, etc. I don't think this is wasting time.
For two, I am a Heavy-enthusiast as a Detective. If I'm a Detective and have enough credits for Heavy, I will totally buy Heavy. The 5-second rule doesn't mesh well with Heavy.
I think this proposed rule would make sense if there were issues with people abusing the "freeze" command as detectives. However, I play/played a lot on this server and I have not witnessed this being an actual problem.
On a related note, I have a question about a rule clarification regarding asking someone to freeze when you are a Detective and don't have a tazer. Example situation: I am a Detective and become HeavyBoi. I give Player1 my tazer, and then I tell Player2 to freeze so that Player1 can taze them (with my tazer). In this situation I do not have a tazer but I KNOW Player1 has my tazer because I gave the tazer to Player1, and Player1 has not left the area, nor used the tazer, etc., is it okay for me to tell Player2 to freeze? I was under the impression that this would be a reasonable request.