<p>Structural frustration is believed to play a key role in suppressing crystallization and promoting glass formation, yet experimentally quantifying and understanding the structural frustration in glasses remains highly challenging. Here, we systematically investigate the structural evolution of a prototypical metallic glass-former, Cu₄₆Zr₄₂Al₇Y₅, during its glass transition and crystallization upon heating, using in-situ high-temperature high-energy synchrotron x-ray diffraction in combination with conventional and ultrafast differential scanning calorimetry. By probing the kinetic interplay between glass transition and crystallization over a broad range of heating rates, we identify a heating-rate-dependent crossover in the nature of the first exothermic process: at high rates it is associated with conventional-like crystallization, whereas at low rates it corresponds to a polyamorphic-like amorphous-amorphous transition. Quantitative analysis of the evolving medium- and short-range order reveals that these structural evolution pathways are governed by the competition between locally favored ordering and extended crystal-like ordering, providing direct experimental evidence that such structural competition can frustrate crystallization and stabilize the glassy state. These findings provide fundamental insights into the intricate relationship between structural frustration and structural stability in metallic glasses.</p>