That's pretty good evidence:)

I tried several different ways to re-create the problem and the code without the .= '' was always approx. 40% faster than that without.

If you want to try and track the cause down, then as Abigail-II suggested above, take a copy of your unfixed script and slowly remove stuff around the salient bit until the problem 'goes away'. If, in the course of doing that, the cause hasn't become obvious, post the reduced version that shows the problem, and let the 100's of pairs of eyes here feast upon it for a while.