Hi
I am getting similar issue on Xbox for most flights. It presents a little different in that the whole sim rate slows down, it’s still smooth but, for example I might set a climb rate of 2000 FPM but in a real time 2 mins I only get 1800. And sure enough when I measure the UTC clock against real time it’s running 20% too slow.
Everything is ok when I take off, hour later on arrival running slow. It’s noticeably slower on approach and landing takes forever.
Although I have experienced slow time in departure also but not always.
I am sure this was not happening prior to SU12. Sorry I have not provided a more technical response I am a more casual user on XBOX but my Brian is not lying to me !!