Well, I've tried to copy and paste about 250 pages' data (image and text) (from one document to another), it took about 3-5 second to finish the process.
So, I suppose the amount of data is not the problem.
Try follow the instructions in the following support article to troubleshoot the problem
http://support.microsoft.com/kb/921541
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BTW, if you are copying these data from website, try paste them with Paste Special - Keep Text Only.
You haven't mentioned the version of Word and that's important because if it is Word 2011 for the Mac then the problem might be Smart Copy Paste that you'll have to disable.
If these are full documents you might try using the Insert > File function instead of copy/paste. Again, depending on your Office version where you find that function on the ribbon or toolbar varies.
I can't do it with one letter, let alone large data. It pauses on a simple highlight to cut and paste or spelling error.
Which version of Word are you using?
Does it make a difference which source you are copying from, or does Word freeze even if you paste a single letter from one Word document to another?