A new framework for generating test sets with high Test Efficiency (TE) for critical Path Delay Faults (PDFs) is presented. TE is defined as the number of new critical PDFs detected by a generated test. The proposed method accepts as input a set of potentially critical PDFs and generates a compact test set for only the critical PDFs (i.e., non-sensitizable PDFs are effectively dropped from consideration), whilst avoiding any path or segment enumeration. This is done by exploiting the properties of the ISOPs/ZBDD data structure, which is shown to efficiently represent a set of critical paths along with all their associated sensitization test cubes. The experimental results demonstrate that the proposed method is scalable in terms of test efficiency and can generate very compact test sets for critical PDFs.