SubDomain for API or a new Domain?

Hello Xanors.
My app it's in beta, just some friends testing.
It's better update my instance to the final domain before lunch the app.

I have my domain in GoDaddy, my plan is set my custom domain as a sub-domain, like api.mycoolapp.com, and leave my web application in mycoolapp.com

What are your thoughts, do you think I can have any DNS performance issues?
Or should I have different domains, one for API and other to the Web Application?

Thanks. 🤙