Bizarrely, adding the missing dimension sped the whole thing up. Result!

README.md

@@ -103,10 +103,10 @@ Basic usage then looks something like the below:
  /* $packedBox->getItems() contains the items that fit */
 ```
 
-BoxPacker is designed to run calculations as efficiently as possible, the 7500+ tests in the test suite run in 11
-seconds on my workstation, giving a rate of approx ≈680 solutions/second which should be more than sufficient for
+BoxPacker is designed to run calculations as efficiently as possible, the 7500+ tests in the test suite run in 9.8
+seconds on my workstation, giving a rate of approx ≈765 solutions/second which should be more than sufficient for
 most e-commerce stores :) If you do wish to benchmark the library to evaluate performance in your own scenarios, please
-disable Xdebug when doing so - in my experience the unit tests take 32x longer (11sec->360 sec) when Xdebug is loaded.
+disable Xdebug when doing so - in my experience the unit tests take 28x longer (9.8sec->275 sec) when Xdebug is loaded.
 
 Requirements
 ------------