Here is 3 suggestions: http://www.fusonic.net/en/blog/2014/07/09/three-steps-for-fast-entityframework-6.1-first-query-performance/
Using a cached db model store
Generate pre-compiled views
Generate pre-compiled version of entityframework using n-gen to avoid jitting
I would also make sure that I compile the application in release mode when doing the benchmarks.
Another solution is to look at splitting the DBContext.If you have a large number of entities then it is a lot and it should be nicer to work with smaller chunks. I haven't tried it but I assume it would be possible to build the models one by one meaning no single load takes 15s. See this post by Julie Lerman https://msdn.microsoft.com/en-us/magazine/jj883952.aspx