We've had the opposite experience - it's been slow, clunky, and hard to learn/teach. Of course, seeing some of the other things the former admin/advocate for it did after she left, I'm not sure it's Grafana's fault.
As far as this type of tool goes Grafana is pretty solid. It might be easy to conflate it with the datasource it is pulling from.
Yeah, we’re trying to make datasource query latency more obvious soon. It’s too easy to say Grafana is slow right now. We want users to make better decisions on what to optimize in their monitoring stack. That being said, dashboard search can be slow on bigger instances. But we’re working on it.