There are no elements in the page for style to be recalculated, so I doubt the reflow is what's slowing it down. I'm curious on what could cause bad performance

There are no elements in the page for style to be recalculated, so I doubt the reflow is what's slowing it down. I'm curious on what could cause bad performance for you though. It runs smoothly at 60fps on an iPhone 11 and the iPad, using Safari. Did you open the standalone page at https://wwpzj.csb.app/ https://wwpzj.csb.app/? My only guess would be that the CodeSandbox UI is too heavy for mobile. For science, I went ahead and converted the code to ES3 and ran on the 10-year old first generation iPad (with setInterval) and it's still pretty smooth.