replace Raspberry Pi with Arduino

I’m building a drone with autopilot and until now I used a raspberry pi, I wonder if i could replace it with an arduino to reduce start up time and cost, is it possible and into what potential problems would i run if its possible?

It depends on what you used the RPi for.

I’m mostly using the Pi to send mavlink messages to my pixhawk, so I guess what I’m asking is: is the arduino version of mavlink as stable as the one for Pi’s?

I would say yes, but for arduino you have to compile your own code in C and not in Python.
Plus, I recommend to use a Teensy 3.2 or 4.0 instead of arduino, same form factor 10 fold power.

thanks for the help, I thought about getting an ESP32, would you say a teensy is better for this?

If you don’t need wifi, then yes, teensy is better.