See this is where a lot of people are freaking out, it does not add up to a 1.25 second delay. Now this is something i learned only because i have played him, so its hard for me to explain it to those who have not played him or did not learn about the timing. After a bit i learned how to land W without actually stunning them (that or they immediately flash away). I would place me stun so that it would force them in a certain direction, normally i would try to make it so they would run to the side. Why would they run? because if you place it so they are close to one side they will try to run instead of flash or use a dash (why use one when you believe you can escape just by running). This is the real trap, because now i know where to put the W, right where the are going to run and i use both roughly at the same time. So normally what would happen is they would get hit my W and if possible my Q, a good amount of times they would hesitate not sure where to go and get stunned, still get hit by my W, then my Q and if it will kill them my Ult, sometimes they would flash or use a blink and get away which was fine due to CD's.
The next time you play the trick a little different, place the stun around them and this time they will either stay in the circle or run around in the circle to dodge the W, it still hit them like 80% of the time and my Q would still hit them either way. These two strategies actually allowed me to hit my W and E most of the time, obviously someone with a lot of mobility is not going to get hit so against champions like Talon or Pantheon i would just sort of guess when they were about to jump on me and try to place my stun before they actually jumped. This was easier with Pantheon as his is more obvious and it actually worked out pretty good against him. Against Talon it was really difficult to land my Stun but i ended up just buying Zhonya's and using it every time he jumped to me then i would blow him up as he ran away unless he went invisible. Ive never fought Lee Sin but i assume its similar to Pantheon, once he hits you with a Q throw down your stun A) he will be stunned when he jumps to you or B) he will not jump and you can run away safely.
Most people will flash immediately, but thats on a 5 min CD and you can do it again really soon. Some people will know what you are planning but for them you develop a whole new strategy. The key is to force them into the stun with your W or force them into your W with your stun, best case is hitting them with both. I know this sounds like wishful thinking and optimism but i was able to pull it off quite well and im low Silver elo.