Hey there, I saw the same thing and it worried me a little bit, but honestly its just ONE core of your CPU capping out. You could either add some threading to your insert script, or just spin up another process and see your insert rate nearly double. Although C might add some speed, I found that in a "realworld" environment where I'll have a bunch of ruby processes (rack apps and the like) it really doesnt matter since its just CPU.
CPU is cheap, but you are going to need some if you want to saturate Cassandra from ruby. Other than that, dont use ruby if you MUST insert fast using one process without threads. :P /thomas
