This does not sound normal. Did you have the performance problems during the first i initial sync from Lightroom Desktop to Lightroom Mobile as well? What kind of edits did you trigger with the Lightroom Mobile app
Could you please send me a LR Desktop diagnostig log - best as a private message with a downloadable dropbox link.
You can trigger the log via LR Desktop preferences -> Lightroom Mobile and when you hold down the alt key you will notice a generate diagnostic log button.