The Issue
I have a CQWP which appears to be configured to run with a CAML query (I'm a complete novice with CAML - I've inherited this set-up). It has been used successfully, until recently, to run a query to show documents Checked Out to [Me] throughout the site collection. The site collection contains one main site (barely used) and 12 sub-sites. There are a total of 7,679 documents that the query runs through.
My theory - I think the number of documents has reached a limit which makes it too difficult for the CAML Query to run? Perhaps there is a document limit? Bear in mind, the query runs over multiple sub-sites and multiple document libraries!
Here is the CAML query which I believe is being used.
RESEARCH
I've found some similar posts on the CQWP-CAML-throttling issue.
The Content Search Web Part (CSWP) is unavailable for me. I don't want to go fiddling with activating and deactivating a feature I don't fully understand the consequences of. The picture you see in this Question is the same as I see in the web part menu. Though this looks like an option for users of SP 2013 Enterprise (on Premises only(?)).
Extra Info
My predecessor left some notes:
Link to the source my predecessor used.
#CAML from 'Checked Out To Me' Content and Structure Report
<Where><Eq><FieldRef Name="CheckoutUser" LookupId="TRUE"/><Value Type="Integer"><UserID/></Value></Eq></Where>
#Line to be inserted in a pre-configured Content Query webpart to list Checked out documents to me: Replace 'QueryOverride' property with this:
<property name="QueryOverride" type="string"><![CDATA[<Where><Eq><FieldRef Name="CheckoutUser" LookupId="TRUE"/><Value Type="Integer"><UserID/></Value></Eq></Where>]]> </property>