I loaded my ROMs onto the 16GB Wintec card played a little golden sun, started a new game and play up to the part where you get to the woods and isac lends you his djins did not get any slowdowns or any other problems.
I am getting between 5MB and 10MB write speed depending on how big the files are that I copy to the card and the random seek was 1ms on hdtach.
I also played a little NSMB, and Mario 64 DS, they played fine also.
When all is said and done I am happy with the card.
The only cart I have is the DSTwo cannot comment on how this card will work on other carts.
First thing I did with the SD was format it with the Panasonic SD format tool from the official SD card website.