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Speech writing is more than applying (traditional) “rules of rhetoric”.
At its simplest, this could comprise checking spelling and conformity with the favoured rules of grammar (there is more than one set of such rules). In some cases, that means fixing errors that could genuinely be confusing, but, in my experience, more often that is removing matters that the intended audience could find an annoying distraction or even trigger classist or other discrimination (“bias”, if you want a softer word).
So, even in this simplest of forms, knowing your audience can be an advantage - especially given that the audience may not be consciously aware of their biases/preferences.
Moving to the traditional “rules of rhetoric”, there are two approaches to this.
The first, and most traditional, is to consider those rules as sacrosanct, and demand that people change their ways of speaking to suit various interpretations of how those are "supposed" to be delivered. This attitude can lead to things such as what is referred to as “received pronunciation”, where television news readers tried to sound English.
The problem with that in a public speaking situation is that, unless it has become an automatic habit, it takes energy to change your natural way of speaking - energy which could be better used thinking about your message and how the audience is responding, and is likely to increase nervousness, resulting in a less effective delivery.
The second approach, which I prefer, is to view those rules of rhetoric as a useful guide, and adapt the phrasing (“ways of speaking”) to those of the person giving the speech.
This does require the speechwriter spending some time getting to know the speaker, which is often not practical. However, the point is important, and should be kept in mind.
With regard to time, one of the "traditional" rules on this topic is that public speaking requires one hour of preparation for each minute speaking, but that includes time considering the details of the topic and points which are typically considered by the speaker and delivered to the speechwriter as part of the instructions.
Nevertheless, some time is required to write a speech, which will be discussed and agreed before accepting any assignment.
In addition, note that I will not prepare materials which are illegal, or contrary to my ethics.
The latter is because, if I am profoundly uncomfortable with the topic, my efforts will be influenced by my biases, and the result will be less likely to be effective - it would be like me trying to write in a foreign language. For the sake of better cost efficiency, I recommend you go elsewhere for such matters.
In addition, if desired I can provide some editing of documents to address readability. However, it should be noted that personal preferences/experience of the audience may be more important than rules of grammar in ensuring good communication.