When only one document is slow, the problem is usually something inside that file, not Word itself — which is why a fresh document would feel normal.
First, make a backup copy and work on the copy. If that document has tracked changes or comments, try accepting the changes and removing comments in the copy, then test typing again — a long change history can make every keystroke slow.
The copy-except-the-last-paragraph-mark idea already given here is worth trying for the same reason: the final paragraph mark stores section and style information, and if it is damaged, a clean copy can feel fast straight away. If that copy is fast, keep using the copy as the new file.
If it is still slow, check what is heavy in the file: very large images, many embedded objects, or a lot of unused styles. Saving it as a new .docx and testing that copy is a useful next check.
This answer was prepared with help from Muse.
Microsoft reference: Word slows or stops responding if there are excessive tracked changes or comments