I find this sentence confusing and unintuitive, and in an interesting way. I have often been told that Russian has double negatives whereas English doesn’t. I.e. “Никто его не понял.” has two negatives and would translate with only one “Nobody understood him.” (i.e. only “no” in the “nobody”)
But here we have “чтобы” which is usually often translated as “so that”, but there is no additional negative in that clause. So I initially misread this as “The professor spoke so fast, so that someone could understand him.”
I guess the internal logic of it is more like a translation: “The professor spoke too fast, (with respect to the case that) someone was able to understand him.” Like…the logic is that the reference point for “слишком быстро” is the whole following clause. Perhaps this is an illustration that “чтобы” doesn’t mean exactly what “that” or “so that” means in English, because “that” is a bit vague. Like, in English saying “so that” usually communicates purpose behind the original clause, and it usually communicates something that either happens, or that the subject INTENDS to happen, like if there is some sort of purpose or intent. Here, it is communicating a specific case which is hypothetical (and did not happen). This is what is so different to me.
I am just going to have to learn this by exposure. It would be helpful if I could encounter more sentences like this. This is the first time I’ve encountered this construction and it is thus very unintutive to me especially given the way Russian usually uses double negatives.