Water splitting reactions through photocatalysis is an efficient and sustainable technique for the generation of green energy. The photocatalyst's ability to effect simultaneous generation of hydrogen and oxygen, along with efficiency in utilisation of charged carriers, conversion of solar energy to hydrogen, fast migration, and low recombination rates of carriers, are the parameters to decide its suitability in water splitting. In literature, comprehensive calculation and analysis of all these performance parameters for a potential photocatalyst are rare. In this work, we have performed first-principles-based computations to find new efficient photocatalysts from the family of Janus MXenes and assessed their performance parameters. Strain engineering has been invoked in search of new materials. Out of 14 studied materials, we find 5 materials: Sc₂COS, Zr₂COS, Hf₂COS, and ZrHfCO₂ under zero or finite tensile strain and Hf₂COSe at 6\% tensile strain meeting the requirements of simultaneous reactions to split water. The computations of various efficiency-related parameters demonstrate that Zr₂COS, Hf₂COS, and Hf₂COSe have excellent efficiencies, significantly better than the well-known photocatalysts. The origin of such performances lies in their electronic and optical properties, which are analysed systematically