It looks to me that while overall your CPU is, as you put it, barely sweating the probable reason why it is struggling is because the test is fully utilising 1 CPU core and Flash is probably not multi-threaded and cannot use more than 1 CPU for the work that is needed.
If you open up task manager and see that chrome.exe is using 12% of your CPU time which is 1/8 your total CPU power then it is the fact that Flash is not multi-threaded that is the problem.
More CPU cores available does not mean more power is available to any one task, see my answer at Clock Speed Calculation in Multicore Processors
I just tried the test on Firefox and on my quad-core system the cpu usage was at about 48%. I'd assume that it is therefore partially multi-threaded, using a worker (calculation) thread and a rendering thread. I would expect your system to therefore see approximately 23-25% CPU usage on the Chrome.exe process.
Weird, just tried it in Chrome and it is only using the 25% (12% for your system) CPU time I originally expected...