Before answering this, I consulted with our Main Oracle person in Engineering, here is his response
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If the customer truly created 4 channels for the restore, then even a partial debug should show if the
channels are all active. I know Oracle does have a setting to limit to 1 channel but this would also
hit the backup I would think. I forget the setting in rman to do this but google.com might find it.
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If Oracle is limiting the number of channels, this may account for the slowness of a restore
Please check on thsi from a Oracle perspective, but, if you can't find anything, open a case, this will need to be debugged