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.