There are various reasons why Idris is slow, but it generally comes down to it being because the current system is the result of lots of experimentation about how to even implement a dependently typed language in the first place, and what it should look like. It needs quite a bit of re-engineering - I prioritised ease of playing with features over efficiency.

The good news is that I'm working on a new version, taking into account the many things I know now that I didn't when starting on version 1. Don't hold your breath though, it might take a while...

In the end, it's a research project, and our job isn't so much to make a good product as to get people excited about the ideas. Mostly we do this by trying to be obviously having lots of fun. But I'd still like a better implementation, because then we can have even more fun.