One performance problem in Idris 1 was due to space leaks in the Haskell caused by using lazy data structures where they weren't needed. Despite our best efforts, we were never able to resolve these fully. No doubt it would have been better to consider where strict data structures would have been better. But, we didn't. This isn't to say that Idris has proved a better implementation choice because it's strict, just to say that we needed to put more thought, earlier, into where we needed strict data in Haskell.