Thanks for filing this. Can you give a bit more insight in when this happens? For example is it with a new airport or only or also with save games? How big is your airport in # stands and # PAX? Does the game get slower when you play during the day, or when your airport grows? Do you use mods or a modified map? What are your system specs?
Thanks in advance, that could really help the devs to understand better. Did you also check this post?