Throw a trainee or re-cert into the mix (especially if it is a relatively rare experience, e.g. in private practice or PPL experiences) then sometimes we all can get too pedantic or enthusiastic about a minor teaching point whilst the airspeed dramatically declines as the academic points are rammed home to the trainee.
An interesting view from another profession where teaching occurs during real life operations.
There are times to be teaching, times to be probing knowledge, times to be discussing procedures et al. On final approach in the aircraft, is not one of them. The trainee would have completed a series of simulator sessions where the process would have been discussed, practiced and then debriefed extensively, if necessary freezing the sim to show/ highlight some area which was causing difficulty. Only when the required standard had been met would the trainee be released to start flying an aeroplane with a Training Captain, Safety Pilot and passengers onboard.
The Safety Pilot is a resource to be used by the Training Captain as he sees fit. He is there for legal reasons until such time as the trainee is seen fit to be able to handle a pilot incapacitation safely. This will require a few days with a Training Captain to ensure his decision making and handling are at a suitable level to divert the aircraft safely and land at a suitable diversion airfield.
He should be briefed before flight to ensure he clearly understands what is required of him. I personally make sure they understand that they are not to 'back seat drive' pointing out every little error and keep interrupting. Some choose to use the Safety Pilot to lower the workload on the new trainee, perhaps by getting them to do the walkround check, or organise the fuelling etc. Others will not.
What any good Training Captain should do though is to clearly brief the Safety Pilot about them being an extra pair of eyes on the flightdeck and to appropriately intervene when safety could be compromised. This incident would be a classic example of when I would require my Safety Pilot to call "SPEED" in a very clear tone until it was acknowledged/ appropriate action taken.
Coming down final approach on ANY sector is not the time to be distracting the PF with inappropriate training input. If a few prompts don't see the situation suitably controlled then you just take control and sort it out yourself. The time to then chat about it is on stand with the engines shutdown.
PP