Without wanting to detract from the engineering accomplishments here (which I have never come close to), it's important to note that low-latency does not appear to be a design criteria i.e. it's okay if it takes a couple minutes to process events during peak load, which means there is some leeway for smoothing the input peaks over processing time.

Furthermore, these are just click events. It's okay to lose a few, so the design doesn't have to be especially good at making sure events aren't lost, as far as I can tell.

A good design is as much about what is left out as what is left in, which is the lesson between the lines here in my opinion.