Simple question, did those of the population who were considered in the half that needed to die via Thanos and his infinity gauntlet, feel pain when they were melting away?
Disclaimer: This answer contains educated guesses and speculation.
Dr. Strange: [You want] to murder trillions.
Thanos: With all six stones, I could simply snap my fingers, and they would all cease to exist. I call that... mercy.
It's not flat out said that the process is painless, but Thanos reassures Dr. Strange that the process is "merciful" when the latter refers to it as murder.
Additionally, no one aside from Spider-Man displays any reaction of panic or agony as they disintegrate-- only looks of shock, bewilderment, and confusion, and occasionally a quiet and solemn expression of sorrow (such as Star-Lord's "Aw, man")
Mantis: Something… is… happening.
Spider-Man: Mr. Stark? I don’t feel so good…
The only counterexamples we have of individuals feeling the Snap happening in any way are Mantis and Spider-Man, both of whom possess some form of extrasensory perception.
- Mantis, an empath, is able to feel that something is about to happen to those around her, though she is apparently uncertain what. Nevertheless, she too disintegrates without any apparent pain.
"Peter tingle"Spider-Sense (though it has not been called by this name in the MCU thus far), which warns him of oncoming danger, could be what gave him such a terrible and panic-inducing feeling.
If we take a look at the people who were vanished didn't feel pain, but they were bewildered by seeing others getting disappeared and then seeing the world vanish before their eyes.
They felt weird, but not the pain.
As per my answer to another question, they felt like they were passed out, which indicate that they didn't feel pain.
Spiderman is scared not feeling pain. He can sense that something is about to happen.
Certainly there are indications that some characters feel something but whether this is actual PAIN or merely a feeling of disintegration / railing against death isn't clear.
Peter Parker seems to know something is wrong...also Drax but others just "dust" without comment or screams.
Indeed, when Peter Parker returns he relates it to like being asleep and then being woken up by Dr Strange. This would indicate that actual PAIN wasn't involved.