That looks bad indeed.

I must say that contracts, while very nice for error reporting and all, also brings additional challenge related to performance (cf https://dl.acm.org/doi/10.1145/2914770.2837630). Our stance is that it's a reasonable trade off for configuration, but the cost would be unacceptable in a general purpose language.

Also, it's true that we've focused on language design and tooling before performance, and then working our way when users report unacceptable perfs on their concrete use-case.

There's been some improvement recently-ish. For example, with the latest Nickel, I get (by inlining `max` to mimic the Nix version):

    $ time nix eval --expr "builtins.foldl' (l: r: if l > r then l else r) 0 (builtins.genList (x: x) 5000000)"
    0,74s user 0,11s
    $ time nickel eval <<<"std.array.fold_left (fun x y => if x > y then x else y) 0 (std.array.generate (fun x => x) 5000000)"
    36,63s user 3,40s

Nothing to be too excited about, though. Although it doesn't make it unusable in small and medium-sized projects (we have users with 120kLoc of Nickel - with they'd definitively like to make it faster, it's still usable), I agree that performance is a big area of improvement, to say the least.