On release, Zed was one of the most balanced champions Riot ever designed, due to reliance on multiple-angled skillshots and dependence on melee range in addition to extreme squishiness. But over time, Riot has slowed down the flow of his kit, including a stealth-nerf in the "buff" that was the increase in his time in his ult's untargetability, which, in reality, just made him even slower. Let's list off everything that they slowed down about Zed, not including the already mediocre speed of his Q (which was meant to be that way, to help it be dodgable)
_ E's cooldown. Went from 3 to 4, nearly identical to his Shuriken's cooldown. Your shurikens are supposed to be something you measure; something you need to be sure to hit before you cast it, due to its slow speed and thin hitbox. So, why does his E, a low-damage melee-range ability, have to have such a lengthy cooldown, on a champion that is built around Energy? Think about it. The intent behind E's design was to be spammed if you have the energy, and if you're in range. How much you can spam it is dependent upon your ability to land a double Q, or a double E, to gain your energy back. So, what sense does it make to make it a 4 second cooldown? I guess it's supposed to try to reduce his split-pushing power, which was extremely strong back in the day. In that case, why not reduce his attack speed instead, and keep the flow and speed and integrity of his kit intact? Oh wait, that is what they did...yet E remains in a nerfed, extremely slow state. Keep this in mind, because you will soon see that Riot has a habit of nerfing things the wrong way with this champion, then nerfing the RIGHT way to get what they were originally after, without undoing the previous wrong nerf. You will see this again when I begin talking about his ult.
_ W's projectile speed. They said that they wanted Zed to think about where he places his shadows, instead of just throwing them willy nilly while making plays. However, all they really did was make it nearly impossible to use W to dodge skillshots, which was about 90% of what made the skill so fun to watch and use. I remember the first day I tried playing Zed after the reduction to his W's speed. I was against a Brand. I could remember always dodging his Pillar of Flame, because he was misplaying and only used it on me whenever I had my shadow up. His bad plays, my good plays. Not anymore. Now, I couldn't dodge his pillar with W unless I happened to have it out somewhere already, which is extremely unreliable, on a champion that isn't meant to have unreliability except in his Q. I got hit with it every time despite the fact that this Brand kept using his Pillar on a target that had his dodgability and mobility open. Except this time...the target didn't have it open. He didn't have that ability anymore. Brand was just doing the first thing he thought, with no heed to my ability to dodge or not. His misplaying, my....inability to make a good play. Riot...you know...for people that like to make articles about "rewarding good play" you seem pretty hypocritical in your balance changes sometimes. Additionally, this serves to further slow down Zed's kit, reducing his integrity as a fast, speedy, reactive ninja.
_ Ult. Remember when I was talking about E, when I said Riot likes to nerf things the wrong way, then do it the right way, without undoing the wrong nerfs? Here we are again. Not only does the untargetability increase slow his kit and flow down even further, Zed no longer has easy ways to regain his energy after he ults, since the shadow spawns where he ults rather than behind the target. Since his E's spam is so dependent on his energy upon engaging, and his ability to regain it...why does the cooldown remain in a nerfed state, despite the fact that he no longer has easy ways to get his energy back? Think about it, seriously. It's to the point where using W to try to get your energy back ends up being a Noob Trap because the energy spent in casting W can give you a net loss in energy regain because of where they moved R's shadow. Yet, despite this, Riot doesn't bother to give Zed more depth in choice, because they are under the belief that buffing a cooldown would buff the character overall. No. Zed is such an intricately made champion. With characters like him, sometimes decreasing a cooldown or increasing some numbers will offer more choice or more intricacy rather than simply making the character more strong and that is what Riot fears, preventing them from making changes to this champion that need to happen.
_Ult and W When The R-back works it's extremely annoying for the enemy team and involves no skill aside from just pressing R. Reworking his ult back to the way it used to be will increase his skill cap and risk and reward altogether making Zed a more balanced champion and making him a lot less annoying (Whee I pressed R and escaped WHEE I'M FAKER WHEEEEEEEEE!!!) I miss when escaping with Zed actually look skill. Can we please make it that way again by reworking his ult back to the way it used to be? 0.35 second untargetability, Zed reaches the target within that time, Zed appears in front of the target with the shadow appearing behind them. If this change were to occur, the W shadow's speed would HAVE to increase, since Zed can't dodge a skillshot immediately after he ults (since his ult shadow is behind the enemy instead of back where he ulted) so he would need that travel time gone for him to be able to do what he needs to do if he's played by someone who knows what they're doing.
As a final note, I'm not saying Zed is "underpowered" but he just doesn't feel right to play as anymore. That hasn't stopped me from playing him of course; I love him dearly. I just feel like he can afford having these bits of extra power I mentioned without being pushed overboard. His ult suggestion is more of a nerf than a buff however, and it would really require his W speed to increase in order for him to have a chance if it goes through.