Teensy is nowhere close to most power efficient options out there. A power efficient arm can be running on micro or even nanoamps in sleep mode. It will of course depend on the rest of the circuit surrounding MCU.

I use STM L4 for my projects.

https://www.st.com/resource/en/product_training/STM32L4_Syst...

On page 4 it says down to 30nA with I/O wakeup, 350nA with 32kB memory retained.