Being a Java developer what do you think, "Would multi-threading play a vital role in maintaining concurrency for efficient and fast transactions of ATM or will slow down the ATM services by making it more complicated?”.

No

Explanation:

Mutli-threading allows for various tasks to run simultaneously this can increase speeds but it would not play a vital role in maintaining concurrency for efficient and fast transactions of ATM. That being said this is not the most important aspect of ATM services, that would be Robustness and provable correctness and these two factors are much easier to achieve using a single-threaded solution.