The convergence between consumer electronics and critical real-time markets has increased the need for hardware platforms able to deliver high performance as well as high (sustainable) performance guarantees. Using the ARM big.LITTLE architecture as example of those platforms, in this paper we report our experience with one of its implementations (the Qualcomm SnapDragon 810 processor) to derive performance bounds with measurement-based techniques. Our theoretical and practical analysis reveals that some hardware features may not suit critical real-time needs, and restricted specifications and buggy documentation creates serious difficulties to derive WCET estimates for software running on this platform. From the lessons learned, we identify several main elements to consider to effectively consolidate the sustained performance needs between mainstream and critical real-time markets.