The impostors manager is now working well. I have also implemented a priority queue, to update "close" impostors more often than far away ones. It's pretty much flawless as long as you don't move too quickly (= at no more than Mach 10).
I have added a save/load feature to my clouds generator, so now loading the program only takes a few seconds. I have also tried to vary a bit the cloud spaces used (the same type was used before); now there is up to 5 cloud types used and randomly tiled over the terrain grid.
The framerate is now pretty consistant, i hardly get less than 60 fps when moving quickly, and up to 100-150 fps at lower speeds.