Hi,
this is as expected because your system is waiting on a valid network connection as well as the time sync to be completed. However you could bypass both checks. Just go to /DietPi/dietpi.txt and set the following 2 lines to 0.
# Delay boot until network is established: 0=disabled | 1=10 second wait max (default) | 2=infinite wait
CONFIG_BOOT_WAIT_FOR_NETWORK=0
# Network time sync: 0=disabled | 1=boot only | 2=boot + daily | 3=boot + hourly | 4=Daemon + Drift
CONFIG_NTP_MODE=0
Now your system should boot quite fast as it doesn’t care about valid network connection or time sync.