Why is the performance of my app perfectly fine on desktop but it messes up on phone using ReactJs?

Someone asked me to create a website for his sportsclub, i decided it try and make this webpage in ReactJs.

https://complexbjj-2a19b.firebaseapp.com/ this is the outcome, looks pretty and works well on desktop.

It's pretty responsive layout wise.

So i was pretty happy untill i decided to build and deploy for production.. Sadly i found out that the webpage wasn't optimised for phones at all, leaving random blank spaces.

I tried allot of things, changing to PureComponents, Using React.memo.., etc etc..

Nothing seems to help, so now i'm here after hours & hours of trying to find a solution with my hands in my hair..

Does anyone know what could be causing this problem?