Comentário oficial
You may already have an idea where this is going...
Your computer is almost 10 years old (the 5,5 model was introduced in June of 2009) , and unfortunately doesn't meet the minimum system requirements for the hardware (Intel i3 processor), nor the operating system (Sierra).

While the software may still run on your computer, at this point, my conclusion is that your computer is simply not capable of properly running Serato DJ Pro and thus any issues you're having are outside our scope for support.
Sorry to say, it's time to upgrade the computer.
