BI Publisher reports now timing out (previously working)

Summary:

I have two BIP reports that were created several years ago that are now timing out. They began failing around the beginning of September 2023.

Content (required):

Two reports created by a third party having been running successfully for several years have now begun to time out. I archived both of the reports into our development environment which contains data approximately 6 months older than production and the problem exists here also, so I know it's not a data volume issue.

The query is very complex and poorly optimised however I want to try to understand what changed before considering a full refactor.