Performance of serial communication when using Arduino rosserial?

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I collected the data from a sensor (sampling frequency is 100 Hz) with Arduino serial communication (without using ROS), and now I'm trying to collect the same data from same sensor using ROS (rosbag) with Arduino Mega on the Raspberry Pi 3. However, when I plot the data collected, it seems to be a little bit quite bad compared to when I didn't use ROS.

I'm wondering if using the Arduino with ROS is not reliable for the frequency 100 Hz (I mean sometimes it may send wrong data), or is it because of Raspberry Pi 3 is not powerful enough to run ROS reliably?

I'm just asking to know if the problem is from ROS or Raspberry Pi side or not, so I can identify the root of my problem.

Thank you.