It is no surprise that the Athens implementation in Amber is much slower than the implementation in Pharo. Athens comes with a demo application that repeatedly draws a complex vector image (consisting of many paths) and shows the frames per second (fps).
In the tutorial in Amber, you can start this benchmark by executing step32 (remember to execute step2 before!). When you click "Do it" a second time, the execution stops and print the fps to the Transcript (printing to the Transcript does currently not work correctly in my Amber version).
If you want to run the demo in Pharo, you have to execute "VGTigerDemo runDemo".
On my computer, I have 30-33 fps in the browser (Chromium 24) and 70-80 fps in Pharo. The demo is slower in Firefox (20-25 fps). I ran the benchmarks on Ubuntu 12.04.2 on a computer with an Intel i5 dual core CPU.