loading – How does DiRT Rally load levels?

I recently played at DiRT Rally and I noticed an occasional shift spike. It bothers me because my PC is perfectly capable of rendering the game at 60 Hz. This leads me to think that it may be my hard drive that slows down the game.

Does DiRT Rally charge assets at one level before starting to drive, or in sections during the race?

If it's loaded into sections, moving the game on my SSD will hopefully remove those spikes.

One thing to note is that RAM is not a limit for my system (24 GB), so I can not believe that the game has a hard time holding a scene in memory.