I suspect most of us have experienced that issue at one time or another. There can be many causes of the slowness: network is bottlenecked, hardware issues, software conflicts, crazy large files sizes.
The answer to your question is in the archive of this forum under the heading "Large assembly".
The answer to your next question is probably also in the archive.
It is true that you need to know the appropriate keyword to search on, but I learn those by taking the time to read other people's posts on the forum.
Define large assembly?
How much data is being moved across your network?
What part of the file open process is slow? Moving the data across the network or after the data is moved and SolidWorks has to work to resolve the files and open?