If a simulator fills on the same bar that produced the signal, it has quietly given the strategy information from the future. The resulting curve is not optimistic, it is invalid — and the effect is largest exactly where an idea looks most attractive, because sharp reversals are where seeing one bar ahead is worth the most.
So the default fill is the next bar, and the clock is explicit: signal, route, exchange acknowledgement, broker acknowledgement, then fill. Open, close, mid or volume-weighted are legitimate choices within that. Using the signal bar is not a choice, it is a defect.
The second killer is costs. In India they are not one rate you can apply as a haircut at the end. Securities transaction tax, exchange and regulatory charges, brokerage, stamp duty and goods-and-services tax all differ by product class — options do not pay what cash pays, and cash does not pay what futures pay. A high-frequency intraday idea can be entirely composed of charges, and you cannot see that if you model them as a flat percentage.
Applying both by default, rather than as options a researcher can turn off, is the difference between an engine that measures and an engine that flatters.