I'm not entirely sure what do you mean by creating tar of microSD content (32GB, as you've said) and where you expect to save such big file (because it is what that command you've mentioned mean).

Nevertheless, problems like this seem to be related to swap problems - try using swap on microSD *only* and search for optimizations related to way swap is used (posted somewhere in easy debian, for example).
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Otherwise, kernel on it's own doesn't change your frequencies, especially into lower ranges

/Estel