I´ve tested on my Galaxy SII with a Kingston 16GB Class 10 SD and it slowed down the system.
The most significant problem was in reading 1080p videos from my SD where before the App were runing smooth and after the App (Configured with a 2048K Cache size) started to chopp and freeze the video.
Just to let you know, maybe is a device especifics and could help to understands why this happens.
Samsung Galaxy S II - I9100
HTC HD2 - Leo
Sony X10 Mini
All Modded like momma says!