TL;DR: The animation for Templar Channeled skills are longer than their one second cast time duration. If you do not interrupt these (i.e. weaving another ability or weapon swapping), the game will play the full animation and lock you out from your next cast for about a half a second.
Or at least that's my theory. Something is going on that if you don't interrupt these channels, there is a lot of "dead" time in which your character isn't doing anything, meaning you'll fire off fewer skills than you would with normal instant-cast abilities.
I do not know if this is the case with other cast-times like Snipe or Uppercut. I suspect something similar is happening, but I have not used those skills very much the past 5 years so I am not sure if and how they may be acting differently since the Elsweyr expansion.
Changes made by ZOS according to Patch Notes:
Removed the 200ms post global cooldown from cast time abilities. This was done to improve the feeling of using these abilities in rapid succession with other abilities, to allow for a more fluid transition, and less down time between usage. We also made some adjustments to the cast times and power levels of specific abilities, which will be called out on a per case basis in further notes.
Puncturing Strikes: Reduced the channel time for this ability and its morphs from to 1 seconds from 1.1 seconds.
Solar Flare:
- Reduced the cast time from this ability and the Dark Flare morph to 1 second from 1.1 seconds.
- Reduced the damage by approximately 37% to make up for the fact that it can be cast more frequently.
Some relevant points here.
ZOS reduced the damage of Dark Flare and other cast times to take into account that the skill would actually complete is one second and there would be no 200ms post global cooldown (or anything else) preventing the immediate casting of the next ability. This does not happen as intended unless a player precisely is able to interrupt the channeled skill. So if you fail to this and avoid that "dead" time, you're just eating a huge damage nerf without any corresponding benefit.
ESO's combat mechanics are not consistent. If you fail to interrupt an instant-cast ability, you're not locked into a prolonged animation that prevent your character from executing it's next ability. Moreover the timing of a light attack or skill weave is much more forgiving as it can be done at anytime after about 0.5 seconds an ability is cast. In short, instant-casts are easier to weave and you aren't punished with "dead" time and subsequent DPS loss for imprecise timing.
A Templar player in theory could practice to get down the correct timing (although the discordance between the animation and cast time is going to make that difficult), but competitive game-play in which other players and Boss mechanics are disruptive is too demanding for this to realistically and consistently happen. This does not even get into the issue of latency/lag that comes with Cyrodiil PvP, connections from Australia, high volume combat in certain Trials, etc.
All of this makes playing a Templar or using Channeled skills require way too much focus and meticulousness than should be the case, to say nothing of in comparison with the other classes/skills.
Hopefully ZOS will figure out what's going on and make playing with channels be just as fluid as instant-cast skills.