Even quite underpowered phones can boot in ~1-2 seconds if optimized for that. Not everything in the phone will startup in that amount of time (modem, wifi), but it's possible to start to Linux userspace and display fully interactive UI in that amount of time.
Even my e-book reader Linux port boots to UI in ~2s.
Even my e-book reader Linux port boots to UI in ~2s.
It really is just bloat and lack of care.