Not always. Quite a lot of contracts can be statically checked. Unfortunately the only contract checker [1] I have experience with is too slow to be useful on source code that is itself large enough to be really useful.

And in C# at least contracts can be disabled for release builds so that the performance penalty is only inflicted on the developers.

[1] https://docs.microsoft.com/en-us/dotnet/framework/debug-trac...