When dictating into Word documents containing more than a few pages, slowdowns are likely. There are different techniques to work around this problem.
For additions to documents where formatting text is insignificant, you may use the "Show Dictation Box" command, dictate into the Dictation Box, and when clicking "OK" the text will be pasted in the Word document at the current cursor location.
Editing can be done using the "Edit Selection" command so long as the selected text only contains unformatted text.
By using the Word Master Document feature, or by having your own document containing links to portions of your text, you can edit within smaller Word documents.
This does not eliminate the problem, but can often postpone the problem until final editing of a document.
Various web sites discussing Word provide more details for working with large documents. Several are at universities where theses and dissertations are frequently very large documents written by persons who are not yet expert with Word.