It seems that the cheapest way to make your iMac faster is to go to eBay and find a newer model, according to what your wallet says. Anywhere from £100 to £2000 or above, depending on age etc. Check out how much hard drive space you need (because SSD drives are a lot faster and either a lot smaller or a lot more expensive) to see if you can buy SSD drive or HD, check the RAM, and the age.