Actually, it's seems to be true only for some particular cards - Benchmark Results: Random Reads And Writes.
The apps in the ext partition are not small files (typically between 200KB and 40MB), dalvik cache is a little below that, but still usually above 256KB (from what I've seen in my apps).
So higher class card is better.
Yea those results are a mixed bag really, there is a decent thread on xda about it (don't have link atm) and the general consensus is that random r/w tests (4KB files) are faster on class 4 and 2. Apparently the way higher class cards work increases latency, to achieve greater sequential throughput? Also reliability of class 10 cards is a serious point, I've had several cards of different brands die after 6-12 months, could just be unlucky though (wasn't using them for swap either)
Edit: here's the badger: http://forum.xda-dev...69#post15269669 I'm interested in what people think.
Edited by 99x, 23 January 2013 - 11:26 PM.