I need to be really carefull for the time critical parts in this project and I think there is no way I could finish the project without using Interrupts. Even with Interrupts, I have some concerns, that the Nano has enough power/ the frequency that it's running on is sufficient.
Do you think you've provided enough information for us to make informed comments? I don't.
What will be causing the interrupts? How many interrupts will there be? How many external interrupt pins does the Nano/Teensy/Cortex have?