Generally, I think there should be no noticeable difference.
async is used for operations that can take a fairly long amount of time, so adding a small slowdown should not affect performance measurably.
That being said, there are cases where
async can negatively affect performance, especially when you have an operation that is sometimes asynchronous, but most of the time is synchronous. To improve that, C# 7.0 is adding support
ValueTask<T>, which avoids allocations in the synchronous case. Though I don't know if there are any plans to use that in WinRT.