The telegraphing attack hits you if you're still in the red zone when it disappears. The red only appears during the casting time and if you're in the AoE when the timer ends I guess the devs only think it's fair that the attack hits you since the actual attack animations would end up adding another .5-2 seconds to the casting time.
It's not the animation time. You can clear the telegraph area well before the casting time is finished and still get hit, and it's in a rather predictable way so it's probably not lag. If you cast a spell and wait until around 0.2s left and start moving, the spell would still finish despite the fact you moved, which suggests there's that much difference between what your screen says and what registers to the server since moving normally should've canceled a spell immediately. Actually, if you look at it that way, it could be lag after all, but again this is very reproducible so if it's lag, then it is always lagged.
In practical terms this means you've to clear the telegraph area about 0.5s before the spell is done casting. This makes the reflex requirement of the game extremely high, and while it's not necessarily a bad thing (though I'd say the requirement are extremely unreasonable), the timing is misleading and makes you think it's lag.