Speed up the sync of Lightroom

Hello,

I don't understand why sometimes Lightroom is very slow to sync, while in my preferences I uncheked all options to store pictures locally.

On the other hand on the web application lightroom.adobe.com everything is fast.

Does anyone have the best configuration to make Lightroom as quick as possible to make changes?

I recently reset all of my ratings to zero and this caused the whole application to crash and is now unusable... (I should point out that I have 20k photos in Lightroom)