I've checked two similar scenarios and one of them seem to be more performant. So timestamped data is being appended, the first use case is with an OPP and new rows being created every with only one column (there are about 7-8 CFs). The second cases is to have rows with more columns and RandomPartitioner, although every row gets much more than one column appended yet the inserts are relatively uniformly distributed among rows. Yet the first scenario is faster than the second, and the second one starts with good response times (about 20-30 ms) and gradually the mean time increases (to about 150-200 ms). What could be the reason? 
Thank you very much!
Best regards, Daniel.