It can run special chip games like SMRPG at full speed. I think people were saying that it was having a little groveler trouble with Star Fox but I don't have that on my 3DS to test. This is assuming n3DS. I don't have an o3DS to test it.
You won't get fullspeed Star Fox even on an N3DS with PocketSNES, usually 40-50 fps. But it's better than CatSFC.