A simple upgrade of pre-2012 Apple iMac (aka “Aluminum” iMac¹) can increase it's speed by 400% and make it run like a new model.
If your iMac computer was purchased prior to 2012, then you're probably noticing how slow it has recently become, on startup and in general use. How often are you seeing the apple “spinning wheel” as you open applications?
If your computer is a pre-2012 model then it is most likely to have a conventional Hard-Disk-Drive (HDD) , which is now more than 4 years old and has a 50% chance of failing as well as slowing down due to age and an accumulation of old programs, utilities and old operating systems.
True speed and performance is gained by replacing the old HDD with a SSD (Solid-State-Drive) which is what Apple started to do on most of it's post 2012 models.
To find out if or how much your Apple Computer can be upgraded, you firstly need your computers serial number which can be found on the base of the stand on iMacs or click on “About this Mac“.
¹Aluminum iMac (2007-2012)
“Aluminum” iMac models shipped from 2007 until 2012. These systems are made of aluminum but have flat, thick sides rather than a sharp tapered edge around the display. These models can be identified by model numbers A1224 and A1225 (20-Inch and 24-Inch displays, respectively, with a black plastic back) and A1311 and A1312 (21.5-Inch and 27-Inch displays, respectively, with an aluminum back).