Thanks for replying.
It all came about from looking into why our page loading speed was slow.
I ran some pingdom tests and found that the initial wait time was super slow.
That led me to analyse just what WordPress was doing in its initialisation, so I fired up P3 (Plugin Performance Profiler).
It identified 4 plugins that were eating up a lot of resources on page loads, Simple Lightbox being one of them.
So I set up a blank 3.5.1 test server and ran the tests with and without the plugin to see the performance difference.
It was quite noticeable.
Here are the images from the test:
1. Vanilla 3.5.1 site with Genesis, WordFence and XCloner
2. Same site as 1 with Simple Lightbox added
3. Out website before tweaking
4. Running P3 on our site
Simple Lightbox plugin takes up 36.4% of the resources
5. Running P3 again after I removed GOTMLS plugin and optimised JetPack, WidgetKit and Simple Lightbox
6. Removing Simple Lightbox plugin and running pindom again
That's all without any server optimisation.
Hope that helps.