Current data science (DS) code generation benchmarks equate correctness with quality, overlooking execution time differences that span orders of magnitude between correct solutions. We introduce DSEffi-Bench, the first benchmark specifically targeting execution efficiency in LLM-generated DS code, comprising 1,000 instances across 10+ DS libraries with stress-testing harnesses and human-validated references. Evaluating 16 models across 3 tiers, we find that correctness alone fails to characterize efficiency: GPT-5.4 leads in correctness (Pass, 66.9%) but its efficiency score (B|P, 71.7%) nearly matches GPT-5.4-mini (71.6%), which solves 47 fewer tasks; Kimi-K2.5 ranks lowest in correctness among frontier models (40.2%) yet achieves the highest efficiency score (73.6%) across all 16 models. A human-annotated five-category taxonomy reveals that 79.1% of efficiency deficits extend beyond algorithmic complexity to domain-specific root causes, with distinct failure profiles across model tiers and libraries. Two exploratory experiments provide initial evidence that these diagnostics can guide improvement, yielding up to +14.7% efficiency gains via taxonomy-guided optimization and approaching Claude-Opus-4.6 Best@3 in efficiency at 13.0× lower cost via library-conditioned routing. Code and data are available at https://github.com/Albert-Gong/DSEffi-Bench.