So I wasnt going to post this, as many people is aware. But seeing as another companion ress trick was posted. That is FAR worse than this method, I wanted to just get it out in the open.
PRO TIP: Dont relog. Dont let your companion sell junk to revive him. Dont summon another companion. HERE is how you do it the fastest way:
1: Companion dies.
2: Right click companion. Dismiss.
3: Summon companion.
4: Companion now has 100% health, and it only took you 1.5 seconds.
I even use this method if my companion is below 50% hp. Its a 50% health pot in 1.5 sec.
Alkzn