Kobo touch: a "slow" book in turning pages and add/remove bookmarks.

Good morning All,

I have an old Kobo one touch :shy: but is perfect for me.

I am able to scroll through the pages smoothly and add/remove bookmarks.
However, there is one book which is very slow and placing or removing a bookmark is slow. To turn a page can take up to 3 seconds.

Any idea?

Thank you.