Enable Developer options in Xiaomi mobile phone

In settings in About Phone tap few times on miui version and you will enable Developer options. Then in additional settings you will find Developer option.

Tools > Settings > About phone > MIUI version > tap to enable Developer options > "You are now a developer!"

If the setting is already activated you will get this message: "No need, you are already a developer!"

It's possible that after doing a Security full check on Xiaomi mobile that the Developer options deactivate. Reactivate using the above method.

Once activated, to access Developer options go to Tools > Settings > Additional Settings. Scroll down to Developer options.

I use the above, Developer Options, to toggle ON useful-for-me settings such as:

Skip screen lock - Go directly to Home screen from wake
Stay awake - Screen will never sleep while charging

Also:
Mock SD card optimisation - Make your SD card much faster
Don't keep activities - End every activity as soon as the user leaves it
Turn on MIUI optimisation