Tracey: I’m guessing that it is creating an IDLK file, but that you’re using some server or cloud solution (e.g. dropbox) that doesn’t honor it. Lock files often don’t work over the cloud, sadly.

Agreed… there should be no problem with 20 parent/master pages.
I wonder if there are other elements that could be slowing it down, such as footnotes, endnotes, cross-references, very long stories, etc.