Yes, it is extremely long. So what? Mages aren't assassins.
And I meant instant as in the sense that they did not die in one blow.
Your first example took 3 SEPARATE instances of damage (Jhin's health bar decreased 3 times). Your second was a 2v1 against a target with like 80% health only that still took 2 instances of damage. Your third was another that took like 2-3 instances of damage.
Instantaneous would be they instantly died from 1 ability/overlapping segment of abilities. AKA, their health bar visibly decreases only ONCE.
https://youtu.be/_U5EJYKW6js?t=43s Look at this clip where Rengar deletes Tristana instantly. That is what I would call "instant". Now, this isn't the kind of burst an assassin should have. Literally not being able to react at all to the burst itself combo is bad (excluding the fact Tristana was a fucking idiot for being by herself and not pinking/immediately running or jumping or Flashing away.
https://youtu.be/_U5EJYKW6js?t=34s Now, look at this clip where Zed deletes Vayne. It was not instant. There were two separate instances of damage, and Zed successfully dodged Thresh's attempts to Flay by shadowing away. This is the kind of burst speed that is perfectly fine and acceptable, and this is what should be used as the NORM for an assassin's burst combo speed.
And in all 3 clips, the ones that died were idiots. Jhin literally could see Kha'Zix going around the side while STANDING IN FRONT OF HIS OWN DAMN FOUNTAIN and chose not to move back and eat 3 separate instances of damage. In the second one, Veigar was alone for whatever dumb reason and got caught out 2v1, 1 of which was an extremely fed assassin. In the third one, Ezreal was an idiot and got caught out alone and got deleted accordingly. What happened in each of your clips is perfectly fine and should be what is considered NORMAL, IF NOT SLOW.
Yes, Ezreal could have jumped the wall, but he jumped backwards instead while still staying in Rengar's attack range. That was his failure. And his biggest failure was wandering out alone ahead of his team when it became quickly obvious his team was only the slightest bit behind him after watching the rest of his clip. So what? He doesn't deserve a way out of that. He HAD ways out of that. But he fucked it up. So no, he deserved to die that fast.
for ad champs, then they reworked and made some new items 
for ap and hybrid champs. and for the cherry on top of the cake, they reworked the assassins. Now what is see in game is talon, kha, rengar, and shaco on the same team all building full armor pene builds and making the tanks either do nothing in teamfights and being ripped by paper or building 4 thornmails and 2 warmogs to survive longer but not dealing any dmg. Please rito do something else with your game. NOT THIS!!!
getting 60% out from his Q cooldown if the target is isolated? if you get 45% cdr you can use your Q at the same speed as 
uses his Q
