Personal computers have an advantage here: it is acceptable for them not to work when they are not directly used by someone. It means they can be stored in safes when not used and have all the encryption keys securely erased when not used. For example, a screen locker could stop all the processes and erase all the keys from registers and memory assuming both disk and memory encryption. And the locker itself could be triggered by some proximity sensor, RFID, camera, whatever, not just input inactivity timeout.