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Well, let's start with the obvious; your computer is 9 years old. While yes, it has a Core i7, that CPU hasn't necessarily held up against change, especially when it comes to multi-threaded operation. (Check this comparison for your CPU vs a similarly classed CPU from a 2015 and 2019 MacBook Pro.)
If you have ANY other tasks running in the background, and on a 9 year old computer, I'm going to assume that you're not using a freshly installed OS, those are going to cause slow-downs and with any other system resources being used, rekordbox itself is starting to get choked out, never mind the fact the computer is also processing data for the displays of the DDJ.
Two things you can do...
1) Increase the size of the buffer for the DDJ. That will help a little, but the next item will be more advantageous.
2) Purge you computer. Uninstall unnecessary applications, terminate background tasks, disable services you don't need. Make it as sleek as possible.
Another thing to try is to disable the Intel Turbo Boost. This is a known issue for both Mac and PC where the turbo boost function will affect audio output.