There can be no doubt that the easiest way to increase the performance of an old Mac is to replace it’s hard disk with a shiny new Solid State Drive (SSD). The problem with this is that officially Apple only supports TRIM on Apple’s SSDs, effectively removing TRIM support from 3rd party SSDs. TRIM is a system-level command that allows the operating system and the drive to communicate about which areas of the drive are considered unused and thus ready to be erased and rewritten to. In the absence of TRIM, users can see significantly slower drive writes as the drive begins to fill up.
There are tolls people have written to try and get TRIM working for 3rd Party drives, but luckily, with El Capitan, Apple is relaxing the reins and allowing TRIM to be re-enabled for 3rd party SSDs.
To do this, simply open Terminal and execute the command:
sudo trimforce enable
Apple does give you a warning:
This tool force-enables TRIM for all relevant attached devices, even though they have not been validated for data integrity while using that functionality. By using this tool to enable TRIM, you agree that Apple is not liable for any consequences that may result, including but not limited to data loss or corruption.
…but after a reboot (the utility will prompt you) TRIM should be enabled and you’ll be good to go.
As always: Proceed at your own risk. Enjoy.