I'm using Office 2019 on a document created many versions ago. This document is 197,088 words long, so far (533 pages). This is still too large for Word to mark spelling errors? Word has always marked grammatical 'questionables' in blue underlines, but stopped underlining in red with the 'spellcheck having a nervous breakdown' message, long ago...
A document of 200K words and 540 pages is not too large for Word to mark spelling. However, if formatted using direct formatting and with multiple words identified as spelling problems, it may be unwieldy.
What happens if you press the F7 key or tell Word under the Review tab to check spelling?