The channel-focused approach to stream processing reminds me of Heka [0]. It was a contemporary of Samza and Heron, and it was fairly prominent in the early Go ecosystem (maybe 10 years ago). As I recall it, quite foggily and quite a long while later, one of the final nails in Heka's coffin was that channel throughput didn't scale well. Do you have benchmarks for Rill, or is it not intended for high-throughput use cases?
I have some benchmarks in the project's wiki on Github. I can confirm your point: Rill's main bottleneck is channel operations, the library itself adds negligible overhead on top. Of course, for multi-stage pipelines the number of channel operations grows.
To summarize, I believe Rill's performance would be fine for any problem solvable with Go channels. I've used it for a wide variety of use cases - while I'm not sure I can call them high-throughput, I've had pipelines transferring hundreds of GBs of data with no performance issues.