iPhone Sync Is Slow
TL;DR: Turn off the "Show this iPhone when on Wi-Fi" setting in the iPhone settings using Finder in macOS.
I've been having trouble getting my iPhone to sync with a MacBook Air running macOS Catalina 10.15.7. When it syncs at all, it does it extremely slowly. I was syncing over wifi, so when I got tired of waiting I connected it to my MacBook with a USB cable. But it was still just as slow.
I had a hunch it was still syncing over wifi even though it was physically connected via USB. So I went into the iPhone settings in Finder (you can get there via Finder's sidebar -- click on your iPhone there) and turned off the "Show this iPhone when on Wi-Fi". After that, syncing was very fast.