iMac (Late 2013 or later): Understanding the first startup cycle
After you press the power button on the back of your iMac (Late 2013 or later), the computer might appear to not start up.
This is normal during the first startup cycle when the iMac is configuring the RAM power settings to ensure that the computer is maximizing its energy efficiency. This setup period may last up to thirty seconds. This duration will vary depending on the number and capacity of the RAM DIMMs installed. Don't push the power button again until you allow the iMac to complete the process; it will automatically continue to start up once it has completed the task.
Subsequent startup sequences will not include the RAM power setting configuration and the iMac will power up immediately.
Note: Installing additional system RAM or resetting the Non-Volatile RAM (NVRAM) will cause the computer to go through the configuration process again.