I have never taken an hour to get from Stansted to the M25 and I have done the journey often enough having lived in Essex and worked in London.
The view from Farnham may be different, but when I was working at Gatwick three weeks ago I found it to be completely isolated from anywhere and some considerable distance from London, despite being connected to a motorway.
I would wager that Gatwick to London City by road would take longer than the same destination originating from Stansted.
Hmm. 38 minutes for the 32 miles versus 57 for the 54 miles having looked it up.