Invoice Group Saved Search Performance when including Transaction Fields

I am having significant performance issues with Invoice Group Saved Searches when I include Transaction fields...

This is the simple search I am running:

As you would expect, running this search opens the results in approx. 2-3 seconds:

That particular Invoice Group contains 4 Invoices, which are relatively small (1, 2 or 3 lines on each).

If I include the Transaction field to show the items:

Then the search takes slightly longer, maybe 3-5 seconds:

And this performance is consistent as you add in other fields from the Transaction subtable - up to a point.

After various testing (where I thought a specific field may be causing performance issues), I now believe that there is simply a threshold of the number of Transaction Fields that can be included before there is a dramatic drop in performance. I think it is 13 (but I'm not 100% sure) - but once you go past this, the Search then takes over