To add a little nuance to defragster's post.
When you select the Teensy 3.5, there is a drop down menu giving the various speeds. On the Teensy 3.5, the options are:
- Normal speeds: 120MHz, 96MHz, 72MHz, 48MHz, and 24MHz;
- Speeds where the USB connection does not work: 16MHz, 8MHz, 4MHz, and 2MHz;
- Overclock speeds (some things may break): 144MHz and 168MHz
For the Teensy LC, 3.0, 3.1, 3.2, 3.5, and 3.6, the clock speed is set at compilation time. The chip can change it in special cases, but other things depend on the value being constant. For example to write to the EEPROM memory, the 3.6/3.5 have to have a slow clock speed. So the EEPROM functions temporarily lower the speed when doing the write, and then raising it again before returning to the user.
For the current Teensy 4.0 and the forthcoming Teensy 4.1, you can change the clock speed dynamically at runtime.
As an example, I was playing around with the 4.1 beta board that I had today, and the chip seemed a little hotter than I remembered the 4.0 being (Teensy 4.1 will use the same chip as Teensy 4.0). I got out a Teensy 4.0, and it too was slightly hot to touch. I then lowered the clock speed to 24MHz, and the chips were cooler to the touch. The internal temperature difference was about 10F or 5-6C cooler when running at the slower clock speed. I would imagine it also uses less energy.