The old PWM way of commanding the ESCs is susceptible to noise and timing issues.
DSHOT is a form of digital serial communication with the ESC - essentially commanding some exact percentage of output. It’s noise-immune and handles missing data packets and all sorts of things.
This is the preferred way for a flight controller to work with ESCs now. There is also some other good methods like CAN and onewire, but that’s another story…
The DSHOT rate (DHSHOT300 or 600 and so on) is equivalent to the baud rate in serial terms. So if you want to send commands to the ESCs at a very high rate (like DSHOT1200) then the flight controller needs spare time in it’s loops to do so, and even that is pointless if the the EKF and motor mixer are not requiring any different output from the motors (so DSHOT1200 could just be sending the same motor output commands over and over and over…)
DSHOT600 is even overkill, but it seems to be compatible with more ESCs, and it seems to slip in nicely with the arducopter loop rates.
If you can use Bi-Directional DSHOT the ESC can send RPM data back to the flight controller without any other wiring.
Full DSHOT telemetry data can be accessed by hooking up the ESC “Telem” wire to a serial port Rx pin and some appropriate settings. You get RPM, voltage, temperature and some even do current.
If you’ve got multiple ESCs all their telem wires can be joined into one.
You can also run bi-directional DSHOT and the full telemetry all at the same time.
EDIT:
See this as an example of the telemetry data and it’s importance