@heldegar
The Raspberry Pi Zero only has 512 MB as RAM, the Pi 3 for example has 1 GB RAM.

So on the Pi Zero I would suggest:
• Do not use a video output in 1080p. Forcing the 720p in the recalbox-user-config.txt is the best way to keep overall resolution "low"
• Disable the Webmanager (it takes too much ram for now)
• Keep the original recalbox theme
• Keep the number of active system low
• Do not use fullsets