Why does Pages constantly freeze on a particular pages document

I'm writing a report (4000 words)on Pages in the past few days. However, since this afternoon, after I added a new picture to the report, this report has been constantly unresponsive whenever I scroll down/up or type something.

I've restarted macbook but it doesn't solve the problem.

I also opened a small document (200 words), everything works fine.

Macbook Pro 2018. MacOs: Big Sur 11.2.3


It seems that the new picture caused the problem.. After removing the picture, everything is back to normal...