When new iOS updates are released, it can happen that even small updates take exceptionally long to download. In my experience the easiest fix to speed up the download is to delete the partially downloaded update and start the software update again. To delete the update in progress, navigate to Settings-> General-> iPhone Storage. Scroll down to the list of apps. There you should have an entry for the iOS update, e.g., iOS 18.1.1. Open that entry and tap on Delete Update as shown in the screenshot. Go back to Settings-> General-> Software Update to start the download again. This time the download should happen much faster. The same fix will work on your iPad, too.