Yes, that's what you want. You can also constrain the Data Value Changes by
Source Web form fillout
for more self-documentation/precision.
The smart campaign works fine and serves my purpose. But the issue is that - there is a time lag of 2-3 minutes between Form Filled on website and receiving the THANKYOU email.
Is there anything I could do to to reduce the time lag?
There's nothing you can do to compress that time, since you're waiting for the appropriate data to be ready -- and email isn't a realtime medium anyway.
Your experience does highlight how strange it is that some people deliberately add a wait step on auto-responders, which just adds more delay. I've never understood that approach and never do it myself.