Vista - Readyboost performance on SD

I found my laptop (Thinkpad X60) has performance problem when I enable
ReadyBoost on a 2GB Class 6 SD Card with the build-in SD Reader.

In my case, I found when I use foobar playing music got many delay / choopy.
I've everything I know and finally got the source. The internal SD Reader
make too many interrupt.

Here is my test
When no SD Card, Processor Interrupt per second is around 2.1K/s
When with SD Card in internal SD Reader and readyboost ON, sometimes up to
~13K interrupt/s !!!
When Readyboost Off and formatting the SD Card, Interrupt/s around ~13K
interrupt/s !!!
When SD Card on my USB Card reader formatting below 3.6K interrupt/s
When SD Card on my USB Card ready running Readyboost, ~2.3K interrupt/s

It seems Readyboost only test Random access speed of media is not enough,
the interrupt created by readyboost must account also.