How do I do them so quick?
I literally take everything out of the oven, fan, top element, bulb cover, the lot. I CAREFULLY paint the inside of the oven with Vanquish. (Back, sides, bottom then roof) I then take what can be dipped [nothing aluminium] and pop everything in the dip tank... [I turn the dip tank on before I even knock on the customer's door ] Back at the oven I paint the removed door and glass with Vanquish then re-agitate the inside of the oven with a stiff paint brush.
Then I work on cleaning the door, then the dip tank bit and finally I only have the oven to clean.
Using wet microfibre cloths I mop out the oven to get all the emulsified carbon, grease and Vanquish off then I spray with de-greaser then scrub with a copper scourer and a good amount of Astonish paste.
I use a window cleaner's scraper with a fresh blade for the dodgy bits and mop out again.
I always finish the oven off with food grade silicone spray which adds amazing sheen and makes the next clean much easier...

Some ovens take longer, but on average, my first visit is 2 hours and consequent visits about 1 and a half hours. But some single ovens are cleaned in well under an hour, 40 minutes has been my best time but I didn't need to take the top element out and there was no fan to remove.