How often is the data being written to the Serial port?
Every 1 second. That includes writing data to Serial port for Nextion display, then saving all data to SD card (was deleted from code). I don't understand where the problem is. And I included just necessary things in code, everything there is for some purpose and can't be deleted. It's even weird, it sometimes skips printing/saving every 3-4 seconds sometimes every 15-20 seconds. I tried to give it more time so I rised from 1 s to 2s and nothing changed. It's very important to save and print all data each second and it makes me frustrated I don't know where can problem be.