I can imagine it for some constrained use case, but taking your typical RDBMS that's powering a variety of business logic with complex queries, I dunno.

One interesting tradeoff Postgres and MySQL made for efficiency's sake was making xacts not fully ACID by default; instead they guarantee something that's good enough as long as you keep it in mind. Cockroach and Spanner are fully ACID, but that means even if you used those as a single-node DB, it ought to be slower.