I use and administer Sharepoint sites and often work remotely so I think I have have good understanding of how it all works. Your initial post title says Word is freezing when you open the file but in the body of the post you say the freezing happens when you edit the file. These are two very different things.

If the problem is when you are opening the file, the most likely cause is network download speed. When you attempt to access a file from Sharepoint, the file is downloading from the server to sit on your hard drive where it can be opened and edited. Downloads can be sped up by minimising the file sizes or getting a faster internet connection. Super slow opening speeds can also be caused by templates not being on local drives. Where are the templates (attached template and addins) located? Are they on the users hard drives?

If the problem is freezing after the file is open when you are actually editing the file then the cause becomes harder to work out. Revision Tracking can be a big factor in causing editing bottlenecks but you mentioned this wasn't happening. So I would examine some other things to work out where the issue could be coming from.

Do your problem documents contain links to other files?
Are there macros being used in documents and/or templates?

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Andrew Lockton
Chrysalis Design, Melbourne Australia