How to troubleshoot how resource governed query is being throttled in SQL 2012?
An update - I still don't know how best to detect what type of throttling has gone on, but I did find the culprit in my case. It does seem that the CPU CAP to 75% was causing a much higher decrease in performance than a 25% drop should suggest. Raising the CPU Cap to 100% fixed the problem. 

A connect article seems to validate this might be a problem with the CPU Cap implementation itself. 

Its not the best answer to this question, and if someone still wants to answer the original "how to tell what throttling is going on..." question. I'd be amenable. 

If you are struggling to understand why your queries are being throttled so severely, this might be a solution worth trying for it. 

http://connect.microsoft.com/SQLServer/feedback/details/781335/resource-governor-feature-cap-cpu-percent-does-not-work-as-expected