I often use flexbox for tiny things like vertically aligning a checkbox and a label and I still don't know whether this is the correct "scope". Like, can I have

I often use flexbox for tiny things like vertically aligning a checkbox and a label and I still don't know whether this is the correct "scope". Like, can I have tens or hundreds of flexbox containers on the page? Does flexbox even take significant time to process, compared to some other css constructs?