I would suggest you have a look at 3rd party skin objects and modules for starters. There are a large number of underlying changes in dnn7 that can lead to badly coded modules causing all sorts of issues.

Okay, tried making changes to the current site by going with a default skin, removing modules with any kind of external scripting along with header meta, made sure all control panel navigation worked smoothly. Upgraded and the same results, have tried a few different combinations of updating prior to upgrading, however the same results throughout.

I would expect to see errors with a skin or related due to new requirements in DNN7, however currently seems easy to break. Since the application pool is stalled and has to quit, one can't really diagnose what the main issue is. Even with all changes prior to upgrade, the experience of extremely poor performance is the same.

Possibly there is some tweaking required with the upgrade to become more robust with the next version. This version has a number of changes, at the same time has required more time to even try to get the site running than experience with prior versions, usually items you can resolve but haven't been shutting down the app pool.

More diagnosing will be done and looking forward to improvements for the next version.