Pylon until i reach stable sampling rate it is necessary for me to know in every second how many values i record.
You definitely don't need the RTC for that!
Because I plan to process these retrieved signals in matlab.
Completely irrelevant!
When the sampling frequency stabilizes e.g. 80Hz or even better 100Hz i wont need the rtc running so many times in a second.
Excuse the word: bullshit!
Have you ever tried to use the millis() value the Arduino IDE offers you?