I used a Fusion iMac for years. A Fusion Mac has a regular spinning hard drive which contains your files and a small SSD for the operating system. It is a nice combination, but because of the spinning drive, it can become slow. Most people cannot open a newer iMac and install a large SSD like I did, but there is a nice compromise for a speed increase. You can run your Mac stuff from an external SSD drive. This article explains the benefits of moving your stuff (including the OS) to an external SSD and describes how to do it depending on what macOS you are running.