Is there an after-cast delay in League of Legends? Specifically I think I've noticed it on Wukong. When I am using nimbus strike to harass, I want to use decoy immediately afterwards to try and stealth and run out of range again so I can harass without the opponent poking back at me. However it seems there is about a .5 second delay before I can decoy and in this time the enemy can usually get one hit or one spell off before I can stealth. I wonder if I just had a spike in latency when that happened or if there is a un-noted aftercast delay. Can someone with Wukong test this out for me?
I can't speak for Wukong specifically, but there are cases of "after cast delay". Or, if you're familiar with fighting game mechanics, it's essentially "recovery" time from executing the attack. Once the recovery period is up, then you're able to resume your assault - otherwise, you must wait.
There's two different forms of skill "recovery" that are implemented in League of Legends for some skills.
The one you're probably experiencing is simply that there is a short recovery period after the resolution of the skill where you cannot act. For example (and this may just be what you're experiencing), Nimbus Strike has a tiny delay between when you make contact with your staff and when you can start acting again. There's a short delay after Blitzcrank purges his Static Field before he can start moving, and Ezreal's Arcane Shift has a tiny delay after you arrive before you can use another skill or attack. The logic behind this is essentially an extension of the reason you can't use skills or attack in the middle of the action to begin with - you're still in the middle of using the other skill. It rate limits your ability to chain skills instantly together. Contrast something like Tristana's Rocket Jump, where you can use skills even in mid-air.
The other form is that some champions, namely Sona and Udyr, do have a forced cooldown on all of their skills after using any single one. In their case, this is to simply rate-limit their ability to cast their spells sequentially - no song spamming, and no ridiculous stance chains.
(Was to long to add as a comment on the answer above so added as an answer)
The delay on ability's is technically called a Global Cool-down the time it takes for the spell to fully go off before your next ability is available you can auto attack during this time but not use another ability for .25 to .5 of a second depending on the ability used its why when you cast one you get a slight delay before the next ability goes off (The Term Global Cool-down originated from WoW and in that case is defined as the cool-down which starts every time you start to cast a spell, and it affects all of your spells. There are exceptions to this, however, as noted below. The basic rule of thumb is that if the spell affects the casting of the next spell, it will not activate the global cool-down.) But is the reason why you see a .5 second delay on the next ability you use as for example Wukong but i reiterate THIS DOES NOT AFFECT AUTO ATTACKS ON COOL-DOWNS
Nothing in Wukong's abilities mention an additional cool-down after using an ability (unlike Udyr). Remember that you're target-able while dashing forward, so an enemy with good reactions could target you then.
It could also be a latency issue, or some other hardware related delay.