If you are asking what he meant by "going farther" he meant that copperheads USUALLY will only strike maybe half thier body length, at most from the ground (but don't count on this), whereas some tree vipers have a reputation for lunging from a branch and just barely holding on with the end of thier tail so that they are striking nearly thier whole body length, and doing it FAST. I've had a bush viper I work with do this a few times, and it sure wakes you up...
As far as strike speed goes, you should be considering that totally irrelevant, for now at least. ASSUME that any snake can strike faster than you can react, because even the slowest can get you if something distracts you for a second. With practice you will train yourself to ignore distractions whenever you have a cage open, but for now, assume it will happen and factor that into your handling, which means keeping more distance than you think you need.