I used it a few years ago with my former company. It effortlessly injected 10K rows of data per second, and we configured Druid over 50 dimensions and about 20 different metrics. In that configuration, Druid was able to respond to most queries within a second. I am not sure how it works but it is an amazing product. We are evaluating it now for use at my current company.

To be fair, 10k rows per second is extremely slow. Clickhouse can do millions per second on a single server in our tests.

We use it a lot at Twitter, though it's a fairly recent addition to the stack.