The performance of an ArrayList in production say for example 1 million transactions will depend on several factors, such as the size and complexity of the data being stored, the type of operations being performed, and the hardware and software environment in which the program is running. It is difficult to give an exact answer without more specific details about the use case and the requirements.
However, in general, an ArrayList in Java provides good performance for most use cases, especially when the majority of the operations involve accessing or modifying elements at the end of the list. The ArrayList has constant-time O(1) performance for accessing elements by their index, and good performance for adding or removing elements at the end of the list.
If the operations involve frequent insertion or removal of elements from the middle of the list, the performance of the ArrayList can degrade significantly because all the elements after the insertion or removal point need to be shifted. In such cases, a LinkedList may be a better choice, as it provides constant-time O(1) performance for adding or removing elements anywhere in the list, but at the expense of slower access time O(n) for accessing elements by their index.
In any case, to ensure good performance for 1 million transactions, it is important to use efficient algorithms and data structures, optimize the code for speed and memory usage, and consider using multi-threading or parallel processing techniques if possible. It is also important to perform regular profiling and testing to identify and address any performance bottlenecks.