"Phone is Starting" screen is displayed during Boot on Galaxy S10
Google's enhanced security policy on Android 9.0(Pie) now enables file based encryption by default for devices secured with a PIN, pattern, or password. During the boot process, the phone will display "Phone is starting..." and the screen may be blank for a few seconds before the Home screen is displayed.
Moreover, as *FBE (File Based Encryption) activated during booting has being newly applied on Galaxy S10 series for the first time, you may feel that your Galaxy S10 series takes more times to complete system booting than other devices.
Samsung does our best to find optimised booting procedure and apply it to new software of Galaxy S10 Series at earliest possible.
*FBE is one of Google security solutions.