Some of you might have noticed that Arduino becomes unresponsive or really slow to respond when it sends information using serial interface.
This can also lead to messages like “Serial port ‘/dev/ttyACM0’ not found.” when trying to upload new sketches to Arduino.
I noticed this years ago but never found a solution because I always somehow managed to get it working after many upload retries and I could not find any information regarding the problem.
So, yes, it was a firmware problem and you can fix it by flashing more recent firmware that can be found here.
And instructions on flashing it are here.