Calculating DC motor rotations from current readings
To get good waveform fidelity you need to sample at ~10 times the highest frequency of interest. At 10 ms per sample your effective bandwidth is only ~10 Hz. With a 12 segment commutator that corresponds to just 50 rpm at the motor. Actual motor speed will probably be thousands of rpm, so your sample data won't accurately resolve the current spikes.
Sample at the highest rate you can, and send the data as raw 8 bit integers without timestamp at the highest possible serial data rate (you can infer the sample rate from the number of samples received per second). At 11520 baud you should be able to achieve ~10,000 samples per second (100 μs per sample). Then evaluate the signal to see if the motor speed can be resolved.