I bought a Lexar brand Class 10 (32 gig) MicroSD from Amazon, and things don't load as fast as I thought they would. I had previously used this card in my HTC Desire Z and noticed that my Gallery would load very slowly. I thought maybe it was the Gallery or the ROM I was using or whatever... but then when I moved to my Motorola XT860 (Canadian Droid 3), I noticed the same thing. The gallery loads images / thumbnails very slowly, and sometimes when I take pictures with the camera, the image doesn't fully write to the card (the thumbnail for the picture will show half the picture, and the other half of the picture will be corrupted garbage). Even when I switched my phone over to the Chinese ROM it behaves exactly the same.
I'm thinking the Lexar MicroSD is at fault here... but I would like to continue testing by getting a new one, hopefully made by a better manufacturer/brand.
Hope this helps.