I made with Blender .ac a sphere with 63K vertices, one with 253K vertices and a third sphere with 1051K vertices.
The first was quickly loaded, the second il loaded in about 4 minutes, the third is loaded in about 60 minutes!
A few months ago I did the same test and the loading times were absolutely acceptable, only the third ball took a couple of minutes, the load times seemed linear with the size of the files.
I noticed the problem because, developing a plane, I realized that, after updating the version of the FGFs to 2016.4 and then to 2017.1.0, the model of charging times seem to follow an exponential logic with the size and not more linear as it was before. During this time I have not changed anything in my operating system and drive NVIDIA 367.57 (Linux Mint 18) I7200 quad-core machine with 32GB of RAM. This problem has been lamented by another developer with a Windows 7 machine.
Someone knows something? tests have been done, by others, of .ac file size greater than 5-6 MB?
My impression is that the problem can be partially solved, breaking the .ac files into smaller segments, but unfortunately this is not always possible.
Do these tests is not difficult, I used as a base plane UFO, which I replaced the ufo.ac file with the ball. Moreover, once the file is loaded, the execution times are absolutely acceptable. The sphere with vertices from 1051K, with maximum definition and quality and active ALS, 9-16 fps.
This is the test with 1051K vertices sphere, the speed is 12-16 fps: