For the most accurate upgrade path for your specific device, check the Fortinet Firmware Compatibility Matrix or the section within your FortiGate's GUI.
The biggest enemy of the 60E in the v7.4 era isn't CPU speed; it's RAM. The 60E comes with 2GB of RAM. In the world of modern security processors, that is barely enough to boot the OS, let alone run deep inspection, WAN optimization, and WiFi controller services simultaneously. latest firmware for fortigate 60e