Google App Engine can be used with Java, PHP, Python and Go. The performance of web apps varies according to the language used? or once the code enters the google servers no matter what language was programmed, the performance is the performance that google gives to App Engine.
When Google launched App Engine in 2008 it had support only for Python, then it added Java, Go, PHP and now as the Managed VMs any language.
When you work at App Engine, you feel that you were designed to work with Python, and that documentation and solutions to problems that arise are much more extensive in Python and Java.
I recommend you see this video , where it makes a comparison of the strengths and weaknesses of each language in the app engine. , basically it says that you use the language in which you are most productive, but in my experience Python and Java make use of the app engine functionalities with more anticipation than PHP and Go, but on the other hand Go is the only language that is compiled by what you would gain a bit of speed there (in the video it says that it is not 100% ready for production, but take into account that it is from 2013)
You should not lose sight of the fact that being computed in the cloud, so the application must scale as your user base grows or the computation you make, rather than in language itself would have a greater impact the architecture of your application and especially the type of searches you do in your data, put a lot of emphasis on knowing in depth NDB and the relationship between entities, since this will make a greater difference than the selection of python vs. java vs. Go
Based on my experience with App Engine Google recommends using Python or Java not so much for performance but for integration and documentation, you will even find tutorials in Udacity that they take you by the hand using either of them.
In my case I have used Java with Spring MVC and everything works perfect, but you must follow certain good practices when you use frameworks like Spring because the very architecture of App Engine that generates instances of your app on the fly depending on the demand, requires that the initialization times be short so as not to affect the users when creating a new instance to balance the requests.
Go is a language that Google is pushing and that should be good, but I think there is not enough documentation yet and one of the key points is how productive you can become with one language or another.
In the end it all comes down to how well you write your code and the good practices you follow, because using the language you use can end up with an undesired product and performance problems.