DietPi services/scripts only run once, when needed (eg: during boot, or when user runs a dietpi-* script). No background processing. Boot time may be slightly longer (due to RAMdisk creation, process tool apply, device identification, network wait to ensure service launch successfully, etc), but overall much lighter post init.
The problem in my case is that the Pi will boot potentially 100s of times per day. It spends most of its time powered off until awoken (i.e. powered up) by arduino supervisor (which is also sleeping, but wakes on interrupt from passive IR). It boots, typically just takes a photo, and then goes back to sleep. It has to work this way to conserve battery. So all the extra stuff (over and above the file system) that runs every boot is just wasting CPU cycles (energy).
Should be sub 25 seconds for Pi Zero, even with network. Which SD card? 17.5MB/s R/W is max regardless of card, however, If no network, and UHS-3 card, overclock SD bus > RPi | Double SDcard IO (Read/Write speeds) . This will vastly improve boot times 35MB/s~.
I am using Samsung EVO+ 32GB, which I specifically selected to reduce boot time since it is supposed to be one of the fastest available. But for some reason I am only getting read speed of 21.3 MB/s and write speeds from 4 to 10 MB/s (write speed is different every time I run the benchmark). This is with a fresh install of DietPi which boots in 26.5 sec.
After adding dtoverlay=sdhost,overclock_50=84 to \DietPi\config.txt read speed increases modestly to 28 MB/s but boot time is still over 26 sec (and the Pi Zero W onboard wifi stops working)