WP7 software RAIDs (kinda) the SD card with the internal flash and treats it all the same. One nice thing about it is that unlike my Nexus One, all storage is first class storage. Apps, music, photos, video - All of it can go anywhere, with no special casing. The bad thing is that if your SD card is slow, everything is slow.

That said, Microsoft should have known OEMs would expose the card slot, and that users would put in slow cards. They should have gracefully handled it, or had the OS ignore/reject crappy cards, like they did with ReadyBoost on Windows.

http://forum.xda-developers.com/showthread.php?t=834144

Confirmed Working(Data remains after soft reset):

Centon 8GB Class 6

Kingston 4GB Class 4

Kingston 16GB Class 4

Microcenter 16GB Class 2

Patriot 16GB Class 2

PNY 8GB Class 4

PNY 16GB Class 2

SanDisk 8GB Class 2

SanDisk 16GB Class 4

Currently Unstable/Unsure:

Kingston 8GB Class 4

Kingston 32GB Class 4

SanDisk 16GB Class 2

SanDisk 32GB Class 2

Confirmed Not Working(Data gone after soft reset):

A-Data

A-Data 4GB Class 6

A-Data 8GB Class 6

A-Data 16GB Class 6

Lexar 16GB Class 6

Transcend 8GB Class 6

Transcend 16GB Class 6

Also, just found Patriot 16GB Class 4 on Amazon, it now claims it works on Samsung Focus

http://www.amazon.com/MicroSD-Memory-Samsung-Mobile-MicroSDH...

When I bought a memory card from Fry's I just picked a cheap 16GB one

Which happens to be that Patriot 16GB Class 4.

I did perform the reset and all of the setting up upon initially putting it in

I do also have a lot of music, my phone has around gigabyte or two of leftover space on it but it should never be this slow.

I was concerned about the lack of upgradable memory before the purchase but it's nice to not have to worry about switching in and out cards and worrying about exactly where anything is saved. I did make sure I purchased a 16MB model though, being stuck with 8MB would suck.