Many factors affect performance, but the two best ways to increase the overall speed of an old Mac or PC are maxing out the RAM and swapping out the rotational hard drive for a solid state drive.
I recently had a client with a late 2009 iMac that was running with the stock 8GB (2x 4GB) RAM and a 1TB SATA 7200rpm HDD. It was definitely running slow and was a perfect candidate for upgrading. The video below shows the before & after results of installing a 480GB SATA II SSD and maxed out 32GB RAM.
Granted, it’s not a highly scientific experiment as there was one additional uncontrolled variable: the operating system was upgraded from OS X 10.7 to 10.10. However, the OS version shouldn’t make much difference when performing basic disk speed tests with a tool like Blackmagic Disk Speed Test.
Opening up the iMac to replace the HDD is not an easy task and I recommend that it only be done by a professional; in addition to unplugging and reconnecting a handful of delicate cables, the fragile glass and LCD panel must be carefully removed and handled properly to avoid costly repairs. There’s also the issue of figuring out what to do with the iMac’s HDD thermal sensor. A step-by-step guide for the 27″ iMac EMC 2374 is available for the brave.