22B transactions a year mean an average of ~700 QPS for the NPCI switch. Of course the traffic is not uniform, it probably peaks at many times that number, but that still doesn't sound that bad - for comparison, a quick Google tells me Nasdaq TotalView ITCH feed peaks at 100k+ QPS at market open.
A great lesson for system design job interviews - if this is a popular payment system in a country with 1.5 billion people, your theoretical system you're designing for a small company cargo culting Google interviews will not likely support millions or even tens of thousands of average TPS.
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