Everybody will learn at different speeds, and have different spheres of knowledge. The most efficient and effective way to learn is at the pace that is right for you. Most importantly, it makes absolutely no sense to learn faster than you are practically able to. The larger a group is, the more likely it is to have people of different ability levels and different learning speeds. A good trainer will pitch to the middle of the group, and the best trainers will be able to differentiate to the edges, but inevitably the pace will be too fast for some and too slow for others.
This is one reason why many people will sit through a course with 20 or 30 other people, and at the end won't be able to do much more than when they started. Most class-based courses require their participants to 'jump' from their actual level of ability up to the level of the theory that is being taught. This makes learning very difficult, if not impossible, but is not the fault of the learner.
I recommend limiting courses to a maximum of 7 participants, and I often have courses with two, three or four participants. The decisive factor is that their levels must be very similar, so that they can learn at the same pace. What that pace turns out to be depends on the individual abilities of the participants. The progression through the language is often quite similar between different groups, but the speed must vary from one group to the next.