Absolutely the best way to breathe life into an older computer is to replace the slow mechanical hard drive with a modern solid state drive. The computer and apps will boot 5x faster, and overall the system will be dramatically more responsive.
Here we have a Mac Mini (Late 2009). It was pulling speeds of around 32–36 MB/sec – this about the equivalent of USB2. A simple, inexpensive Samsung 750 EVO replacement drive improves this to 135MB/sec read and 105MB/sec write. That’s a 3x to 4x speed improvement.