AlwaysOn Async read takes more time when compared to that of Sync secondary
There are a number of different factors involved but it is entirely possible to have a replica that is synchronous be faster or slower than an asynchronous one.

To list a few:


Replica is under too much load
Hardware specification differences (i.e. synch runs ssd and async runs rotational) or synch is using 8 cores @ 3.6 GHz and async is using 4 cores @ 2.4 GHz
Stats and/or indexes were updated and haven't yet been fully redone on the async
Geographical network latency