We've had one other customer with the same symptom Richard. He'd already been through the expensive branded blades and was trying ours as he'd run out of options, but still ours didn't last.
After a number of conversations with him we figured out the product he was cutting, although described as porcelain, behaved more like a fully vitrified glass as it was so heavily compressed.
He solved the problem by dressing the blades he'd already bought.
The anatomy of a blade is diamond pressed into a metal matrix around the perimeter of a steel disc. The metal matrix is as important as the diamond in that it's matched to the product which is being cut. As the blade cuts, the metal slowly wears against the product and releases new diamond to do the cutting.
If there's not enough abrasion from the product the blade appears to dull and slow down, it's not exposing new diamond. This is why in your case the final cut took 3 times the first cut. The metal part of the matrix is not wearing, it's overheating and glazing over, in doing so it "locks" in the new diamond content and stops working.
Dressing the blade wears aware some of the metal matrix and exposes new diamond, this can generally be done with something abrasive like sandstone, concrete or a dressing block specifically designed for the job. You might find with your particular porcelain you're having to do this every 10 or 12 cuts to keep the blade running at optimum performance (whatever brand it may be).
It's worth trying before you throw more money at branded blades and risk the same problem.
Your batch of porcelain is an unusual thickness at 16mm. What brand is it and where did you buy it? It's good for us to know if we have a customer come across the same product/problem.
Example of blade dressing here:-
What is a Dressing Stick and How Do You Use it on Diamond Tools - https://www.youtube.com/watch?v=f-jFJd07yTA