Both apple and Google have people working on optimizing this type of speed test. You wouldn't believe how much pressure the Unity game engine Devs were put under to make loading fast and memory use low. A lot of work was put into letting textures be readonly, pageable and backed by flash when the app is in the background, which dramatically reduces the dirty set size of apps and lets you run many in the background at once.

Therefore, I would suggest it's best to use other kinds of real world tests, like for example completing a specific set of tasks, or webpage loading tests, which aren't yet over optimized.