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 up to 5x faster, and overall the system will be dramatically more responsive.
This is a Mac Mini (Mid 2011) upgrade. It requires a complete gutting to get to the hard drive tray, #alwaysatthebottom. The stock hard drive was pulling read speeds of 98MB/sec and write speeds of 88MB/sec. Replacement Samsung 850 Pro topped that with 519MB/sec read and 348MB/sec write.
This machine acts as a server, deploying Windows Server 2008 via a Parallels virtual machine. Removing the server's data retrieval bottleneck and logging in via Remote Desktop Client creates a pretty smooth experience for the client machines.