Yep; the NSDI paper is correct in that there's hardly any time spent waiting on the network (as our traces in part 2 will show). However, that is not to say that the network being faster cannot help: if computation and communication are perfectly overlapped, then "blocked time analysis" (term from the NSDI paper) would not show any potential improvement, but faster communication can still improve the overall runtime (e.g., by reducing busy polling, or crucial updates arriving sooner).
The CPI number investigation is quite a good idea -- we in fact already have these numbers for the Rust-based timely dataflow, but I'll have a look to see how hard it'd be to get them for GraphX/Spark.