I have to acknowledge, it has probably been more than a couple of years since I have done any real DNN (formerly DotNetNuke) development. Quite a few things seem to have changed in this time, especially with DNN 7.x.
I present below various Tips & Techniques (not in any particular order) for optimization of a typical DotNetNuke installation.I learned these while using DNN over time, while I encountered some of them while browsing over web.
A typical DNN install requires considerable server resources, and creates significant traffic between the database & web server. These techniques aim to reduce both of these where-ever possible, along with some other optimizations. Some of these may not apply to you, and I advise you to exert discretion in deciding which of these work best for you.