Is the transformation a slow and yet painful process? 
Worgen Transformation
We do see in Wolfheart and Curse of the Worgen that it is definitely a physical and gradual change. Unlike ingame, there is no instant magical effect, or dramatic smoke. It involves growing of bones, cracks and pops, and tearing out of clothing as the worgen grows a third in height. They can feel the change, the body expanding, hair rapidly growing, face extending into a grizzled muzzle, and more.
Speed
How much times does the transformation require to complete? In the source material, it does not take all that long, but it does take long enough for one to notice the change and each of the gradual shifting of muscle and bone. It may also be in some way related to anger level, adrenaline making it faster. We see in Wolfheart where the Gilneans shift at the meeting for theatrical effect, and it is slow and described. In contrast we have Halford Ramsey jumping into battle, or Genn shifting to jump in Lord of his Pack, which seem to take less time. However, this could just as well simply be the difference in detail between a shortstory and a novel.
Pain
I would not go so far as to call the transformation painful, but it is shown to be somewhat uncomfortable. The worgen transformation is no mere magical appearance, it is a gradual, physical change, as your muscles shift and bones grow. Characters do not describe pain, and they do not scream through the change like some werewolf transformations in fiction, but some - like Genn in Curse of the Worgen - are shown to be clenching their teeth as the pops and cracks begin. Others do not appear to show discomfort, but are distracted and roused to the fight.
The real pain or discomfort described or depicted in Worgen lore is not when you are changing… but when you are trying -not- to change. In Curse of the Worgen, we are shown Ramsey and Greymane. Ramsey is bitten, and fights the curse for three agonizing days before it finally overpowers him, and he has his first change. Greymane on the other hand is already a worgen, and is capable of human form, but it is painful to try and keep himself from changing when he gets worked up.
Wolfheart (p. 209)
Their bodies swelled, growing a third again in girth and height. Although originally loose-fitting, the Gilneans’ clothing still proved too tight for this shift, and shirts and jerkins ripped loudly. Hair sprouted over the Gilneans’ arms, legs, chests, and faces, spreading so thick that it became fur. Beneath the fur came the sounds of cracking and popping, of bones shifting and tendons stretching into positions of which they should not have been accustomed. Their arms and legs twisted as their forms contorted, the legs turning sleeker, more akin to those of a swift predator. Each figure became hunched, but in that manner of a powerfully built beast. As the audience watched, rapt, the Gilneans’ hands stretched and the nails grew into long, savage claws. Yet, that paled in comparison to the astounding metamorphosis of their faces. It was not just that the ears narrowed and stretched but that the mouth and nose pushed forward, melded together, and created a muzzle filled with sharp teeth capable of rending through flesh without trouble. The worgen stood before the Alliance. The lupine figures held their ground, although there was in them the evident urge to run, to hunt. They did not turn from the gazes of the crowd, instead staring confidently back.
Knaak, Richard A… World of Warcraft: Wolfheart (p. 209). Gallery Books. Kindle Edition.
Curse of the Worgen
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Lord of His Pack
Genn watched the skiffs bouncing in the choppy waves. No. He could not stay. He could not. This was his family. He owed them much. Even now, with their world shattered to pieces, even accounting for every foolish choice he had made, Mia and Tess still believed in him and supported him. He took a deep breath and let out a roar. He could feel the change, his body expanding, his hair rapidly growing, his face extending into a grizzled muzzle.
With a loud howl, arching his back and reaching his arms out to the sky, he completed his transformation. He was a worgen, one of the wolf-men whom he had asked Arugal to summon all those years ago—one of the wolf-men who, with the Forsaken, had inevitably destroyed his nation. But in this form, he was faster and stronger. The curse that he had become afflicted with had its advantages.
He ran toward the starboard side of the ship, full speed. The wet deck didn’t affect his balance: he was singularly focused. The animal instinct inside him coursed through his veins. His mind was hell-bent on the act alone, nothing else, just the doing. And then, as he reached the railing, he jumped!
Thanks for the question! 