So I've been using the Watch on the Tube for a few days now, and I do love the convenience - but it definitely, absolutely, takes far too long.
This is even with double-tapping a good 10 seconds before actually going through the barrier.
There IS a very noticeable 3 - 4 second delay in opening the barrier. At rush hour, at Bank station, that is unacceptable - not necessarily for myself, but for the people behind me. Several times now I've had tutting and sighing as people who are used to the speediness of Oyster wait a few seconds behind me while the Watch opens the barrier.
I think the issue is that Contactless takes longer than Oyster in TFL's implementation - and Apple Pay takes slightly longer than Contactless. Knock on effect of delays.