Frame per Second OPTIMIZATION

Hi all, I'm working on a Virtual Tour of my School and once I filled up the rooms with desks and chairs and start going around the building i have an insane drop of frame from around 80/90 fps to under 10fps. In the Stats tab I can see that when i look forward to certain areas the voice "vertices" goes over 20/22 million. Any ideas or trick on how to optimize the project to have at least always over 30fsp ? The desks and chairs are made with Blender, and I've apllied on them the Decimator function which reduces the numbers of verts.