I had the same experience (along with other issues) with my 7W laser when I initially set it up. @Woodworking_Pilot shared his settings with me on this post:
Help me understand my 7W JTech Laser Test - Accessories / J Tech Laser - Onefinity CNC Forum
After I made the adjustments, my actual burn time has been very close to the simulation time. Currently I have the 24W. I am using the same settings (as posted in the attached link) and my simulation times are pretty close to the actual time.