It's kinda amazing that the people who wrote this probably know every single byte of code that's running on that computer, rom included. Without such deep knowledge you can't get this kind of performance.
With a modern computer that is simply unthinkable.
C64 demo coders don't generally care about ROM contents, there's nothing useful in there. ROMs are usually switched off. Although there might be a few exceptions.
With a modern computer that is simply unthinkable.