Some general tips and this is from pre-AU, but I'm sure it still applies: do the recognition improvement many times in different lighting conditions, different angles, and try to put the phone out as far as possible while it still recognizes you, and with AND without glasses if you have corrective lenses (so it can recognize you in both scenarios, also contacts don't seem to matter as I also wear contacts but have not needed to train it with AND without contacts). Basically the more data it has of situations you are in, the faster it can recognize you.
One scenario where iris scanner seems to struggle with is in direct sunlight outside, but at the same time I never actually trained it in that setting so that could be why. Also, I'm not sure if it has trouble when there is movement. Usually if I'm outside, I'm walking and it seems to have trouble but again I'm not sure if that's more attributed to being out in sunlight or if the shakiness itself makes it difficult for the iris scanner to recognize me.
Since AU, I have personally found the speed/distance needed to recognize me to increase, but for some people they have to put the phone closer to their face than they would prefer. One forum member wrote that the distance he/she usually holds the phone seems to exceed the distance that the iris scanner needs in order to recognize him/her. The distance issue is not as big of a deal for me, but obviously it would be nice if it could recognize me from a longer distance.