I've run some tests on the front end of a simple site in both 6.0 and 7.2 and found the results to be basically identical (and fast), but this is on Railo 4.2, I don't have CF11 installed.
So there is definitely nothing "low level" in the framework itself in 7.2 that would be causing performance issues.
It's either likely to be some specific code (possibly a tag in core, e.g. genericNav, or possibly your custom application code), or it could be related to the CF11 engine (e.g. the "connector" bug where you need to make sure you have re-run the web connector after you have applied updates to CF).
Also remember that you can being to track down slow webskins easily using the Webskin Tracer:
And in Fusion Reactor you can click on a request in the Request History, then go to the JDBC tab and find out if you are running too many queries or some slow running queries:
Ideally if you're doing a true performance comparison you would compare the same app and only vary the version of Core -- comparing 2 different apps and 2 different versions of Core will give funny results that are hard to line up or draw a useful conclusion from.