*Everything* in this game has an inherent delay built into it because of the way the combat system is designed. Skills choose their targets and perform all combat rolls and calculations (hit, dodge, miss, how much damage should be dealt) at the time of activation (either a cast finishing, or the server learning that you pushed the button for instant cast skills). However, they do not actually apply these effects or deal damage until after an animation has played. This leads to a disconnect between providing input to the combat system and receiving a response that stacks on top of the inherent delay caused by Internet latency.
The most obvious with abilities that have long animations. Titan, for example, uses Earthen Fury after the heart phase, where he charges up and explodes after a couple seconds. This is actually an instant cast ability, and the server decides who to hit and how much damage to apply based on their state *before* Titan begins his animation. This is why you can pop Manaward, get a Stoneskin/Adlo, or pop Sentinel while he's charging, and yet you will still take full damage even though there's still another three seconds before the big explosion occurs. Another example is Twintania's Death Sentence. The game decides how much damage to hit for when she finishes casting it. Say the attack rolls for a 5000 hit, and the tank has 5500 HP. You will hear the limit break "SHING!" signalling that someone almost died *before* Twin actually stabs the tank with her tail and removes the HP, which can be used to a healer's advantage to quickly cast Lustrate and get the tank back up to a stable HP level instantly.
This delay affects EVERYTHING. When a BLM finished casting Sleep, at that very moment you were within 5 yalms or whatever the range is on Deep Sleep. No matter where you go or how far you move after that point, you will be slept, even though there's still another half-second of animation delay before the server starts applying the Sleep status effect to anyone who was in the radius at the time the spell finished casting.
This applies to movement and casting interruption as well. You cannot round a corner to LoS a Bard and immediately start casting, because any heavy shots or auto attacks that were "queued up" by the Bard while you were in LoS are guaranteed to hit you, even though at the time the animation actually finishes and applies the damage to you, you have been out of LoS for at least half a second. Casting Surecast or putting Adloquium on myself on my SCH works the same way. I still take damage (and am thus interrupted) from any abilities that were initiated *before* I received the effect of Surecast/Adloquium, even though they hit me after I receive those effects. This is why Paladins and Warriors can still take damage and die for a brief moment after using HG or Holmgang and receiving the status effect.
In PvP, this issue is twice as bad as it is in PvE, because you're dealing with your latency to the server when dealing with position as well as the OTHER person's lag. Let's say you move around a corner at timestamp 100ms. It takes 100ms to get to the server, so at 200ms you're around the corner on the server. But then the server has to tell the Bard chasing you that you're running around the corner, which takes another 100ms. So at 300ms, you finally round the corner on the other person's screen. This ignores that the server only updates positions every 100ms to begin with.
Basically, it results in a combat system that simply does not feel good in PvP situations. Frontline is still fun, but you have to learn to ignore these kinds of things when playing PvP in this game or it'll drive you mad.