Complex sentence: SET could help most with less popular files, it is not believed to improve transfer rates much for popular data, where there is already a huge set of people downloading it, but experiments suggest that in the other cases, SET can help a lot.
Split into simpler sentences: SET could help most with less popular files, but it is not believed to improve transfer rates much for popular data, where there is already a huge set of people downloading it. Experiments suggest that in the other cases, SET can help a lot.