Xtykutl

if you have a controller that only contains this code:

var ThingQueryable = GetQueryable();



Thing ext = await ThingQueryable.FirstOrDefaultAsync();

Is there any benefit over this code?

Thing ext = ThingQueryable().FirstOrDefault();

Is there any benefit over this code?

For client/server code in Windows Forms or in WPF the benefit is that you don't block the UI thread while waiting on the query. That's a big deal.

For Web apps or APIs the benefit is that you don't block a client thread while waiting on the results. For most database applications this benefit is tiny as you won't have many hundreds of concurrently-executing database requests, and so the benefit of not blocking threads is small.

There is also a slight risk that there won't be an available thread when the query is done, and so the method might take slightly longer to execute.

It really depends what solution you are supposed to build.
It the case is simple application that you know will be used by 5-10 users in intranet for a small company then using it you won't gain much benefit (such applications/system sounds pretty outdated for me). Even in this cause i will consider using async service only starting from the fact that it costs you the same time since it is really well supported by framework.

The primary purpose is scalability from server side. So if you are building system that you want to scale you should go definitely for it. If you start using it, it won't magically improve your performance problems - if you have poor queries you still have to improve them.

I like these two posts about showing performance benefit of using it:
Scalable and Performant ASP.NET Core Web APIs and
Async vs Blocking services.

So in short, when start building system and things seems simple you will never know when they will get more complicated (they will probably go)- achieving better performance from step one of project life-cycle can save you lot of troubles later.