I had a day trip to London yesterday, flying into and out of Gatwick. I don't know where the OP gets the idea that the Gatwick Express is slower than the normal over ground trains - I am living proof that it's a darn sight slower.

our first mistake was buying a non-Gatwick express return ticket at the Gatwick ticket office; though about £14 dearer, it would have been well worth it.

Going into London there was no problem -we were in no hurry and the Southern train took us into Victoria in about 40 mins with few stops. our problems came on the way home - instead of getting the bus or tube to Victoria, from the Strand we decided to walk over Waterloo Bridge, and pick up a train going to Clapham Junction and change there for Gatwick. The first train stopped not only at CJ but also at Vauxhall; the Gatwick train stopped EVERYWHERE, including 10 mins at Redhill where the train had to divide. to add insult to injury it then sat for 5 mins outside Gatwick while the Gatwick Express cleared the platform.

we made it to the plane with about 5 mins to spare.

The moral of the story is that if you are going to use the non-Gatwick express trains, make sure that you pick a fast train rather than one that's going to stop at every station.

Contrary to what is said above, IME it would be a good idea to buy your ticket ahead of time as the queues at Gatwick to buy tickets were dreadful, even for the machines.