Evaluating an AST node costs too much MEM?

@nau [reported](https://x.com/atlanter/status/1846574231636656407) some stats and they suggest that evaluating AST nodes consumes 96.66% of MEM, which is an absurdly huge amount compared to the 3.34% that builtins consume. Does it really have to be that way? And if it does, should we prioritize working on solutions making MEM consumption lower by evaluating fewer AST nodes or producing fewer evaluation frames? For example by adding `fix` ([issue](https://github.com/IntersectMBO/plutus/issues/6577)) or `let` ([issue](https://github.com/IntersectMBO/plutus/issues/4797)) to the AST.