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Thanks for your interest in the Intel® Joule™ Platform.
Reviewing the ZR300 hardware requirements System Requirements for the Intel® RealSense™ Development Kit... , it seems that this Camera has only been tested with the Intel® Joule™ 570x, this might be the reason of the slow performance.
Now, I checked the Ubuntu manual Ubuntu Manpage: htop - interactive process viewer and I found out that you can increase the selected process's priority by using F7, maybe this might help you dedicate more resources to your processor while running the SDK samples.
Hope you find this information useful, have a nice day!