horizontal list of 20 item massively slow down React App

I have a list of 20 times, most of the items are just 1 or 2 sentences with some being pictures.

I made them into a flex horizontal list with the intention of creating horizontal transition list.

This massively slowed down the APP.

I know that it is the number of items that is slowing down the APP because when I reduce the list to 3 items, it sped back up.

I have a Xion multi core computer. It can’t be the hardware.

What could be the reason?