Comment by gizmo686

I had a similar problem a few years back. Our decades old system had a poor architectural system baked in that was causing major performance issues in some cases. Fixing it within the core was straightforward, but ended up violating an assumption that almost every module we had made.

After two attempts at fixing it failed (due to getting hopelessly out of synch with mainline), we put the fix behind a flag and setup updated our CI to run the test suite with both modes of the flag; and to only fail for new test failures.