If your Mac display is slow to reach full brightness
If your Mac display is slow to reach full brightness, try these solutions.
Give the display time to warm up.
Make sure the display brightness is not turned off. Press the increase brightness key to check or adjust the level of brightness.
Turn on the ambient light sensor so your Mac can automatically adjust brightness based on current lighting conditions. Choose Apple menu > System Preferences, click Displays, click Display, then select “Automatically adjust brightness” or “Ambient light compensation.” The options appear only if your Mac has an ambient light sensor.