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Have you tried to replicate the issue a few times?? It could be just a game's related frame-drop (they're not Always the same you know, it depends on how much cars were framed and many other things).
It would be weird to have frame drops in game (it never happened to me even from USB).
If it's a SMB loading problem, then it could be a slowdown in loading data, but for cause frame drops the game should be loading in streaming (it does not receive the data on time), but even in that case I think there will be just missing textures or something like that (like with GTA games), not frame-drops.
Therefore it could be a issue directly caused by the patch made for this game??
Have you tried a build pre-smb with already the SRS patch (it was to not make the game to freeze when played after a IGR) or was it a older one without the SRS patch??
EDIT: Wait, now that I think about it, it can happen to have frame-drops (micro-freezes actually) in streaming games, it seems to happen with GTA SA from iHDD i.e.