CPU usage is my first guess. I think the default preset uses a lot of cpu. Try starting Rakrarrack from the commandline and specify a different preset (rakarrack --help for instructions).
If you want to test it fast, open top or htop and watch the CPU usage as you start Rakarrack.
Depending how you have things configured, and what latencies you're using, a 1.7GHz processor might be a bit weak for that default preset.
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