No, the size of the atmx files will not have any impact on the efficiency of your bots. The file could be as big as you like although I advise against doing that as a best practice is to have large chunks of work broken down into small subtasks and run those subtasks using ‘Run task’.
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The things that will impact the performance of the bot is the logic it is following. For example, if you have run your bot in a loop where it gets stuck or has to wait for results will slow your process down other than that you’re good to go.