I have different kinds of constraints, mainly propositional clauses (a or b or c) and pseudo-boolean constraints (2a + b + c >= 2). Long story short, a propositional clause is a special kind of pseudo-boolean constraint, so I can process my clauses as if they were regular pseudo-boolean constraints, but this is slower than using dedicated code.
Lots of problems only deal with propositional clauses, so they take longer to solve than they should, because I cannot say "this is a problem with only propositional clauses", vs "this is a problem with all kinds of clauses". I could use interfaces, and tried, but this is way slower, to the point it doesn't make sense to use them.
And I can't use C++ or rust for this problem: there are already tons of very good c++ SAT solvers, but my goal was to provide a SAT solver I can use in my go programs without resorting to cgo black magic.
But yes, if you're writing performance critical code (which you generally won't be), then you'll need to use code generation to avoid boxing.
This kind of discussion is pointless without some kind of real world reference point. The vast majority of Go projects will never run into a situation where the overhead of using interface{} is a performance bottleneck. If you're working on a project where you consistently have to think about the cache performance of your code, then C++ or Rust would probably be better choices. Go isn't meant to be all things to all people.