An everlasting question within linguistic theory is about the source of locality effects. In particular, a central issue is whether locality effects are driven by syntactic rules or by general principles of language processing. This paper attempts to contribute to the debate, advancing the hypothesis that local constraints in syntactic dependencies are modulated at the interface between the grammar and performance systems. In short, the idea is pursued that both grammatical and sentence-processing factors directly affect the outcome of long-distance dependencies, giving rise to (at least a subclass of) the phenomena that we know as locality effects. Experimental data on children's comprehension of object relative clauses can provide an empirical support to this claim, pointing to this exact two-fold nature of locality. Specifically, it is shown that intervention locality effects in Spanish object relative clauses and their ameliorations presuppose grammatical competence, yet turn on principles of the performance system.