If you mean the main DTM library file referenced from the Header Code, than the answer above is IMHO pointing you in the right direction.
If you mean the libraries of various Tools that you can deploy via DTM than the answer would be this:
There are some (like Marketing Cloud Visitor ID Service or Target) that you really should implement synchronously, because otherwise you could easily break some of the logic.
On the other hand I don't know why Adobe Analytics library doesn't support asynchronous loading like Google Analytics does, because mostly the measurement doesn't have to be synchronous and if asynchronous, it could be loaded higher in the page thus firing sooner in most cases. So I did a little experiment with trying to load the library asynchronously and synchronizing the events manually (because i needed some data that can be found during the page rendering). Result of the experiment is this: http://en.webanalytics.cz/2014/09/01/newest-adobe-tools-configuration/