There could be multiple scenarios causing this issue, which we need to drill down.
Does it happen with any one particular file or even a fresh new file?
It tends to happen on pages that have a lot of text on them - such as a T's & C's page.
Would you mind to share a link with us? I couldn`t reproduce your issue. Edit in Muse or in IBE?
Maybe you are facing slowness due to old cache corruption.
What I can suggest you is resetting the preferences which actually helps most of such issues.
Please refer to this for same - Problems on the very start of using Muse