Can you explain a bit more what you are trying to do, which version of Lightroom you are using and which OS? I sync images to devices all the time and it works fast and flawlessly. Of course it does depend on the speed of your internet connection. And your cellphone may be set to only sync if there is a WiFi connection, not over a mobile connection.
I use Lightroom on my iPhone and iPad, and Lightroom Classic on my computer. I sync photos between these apps and love the funtionality. It works great. The sync is usually pretty quick when I am connected to wifi. Maybe we can help if you can better describe your workflow.