I've always liked it when the weight gain is just slow and natural as I just like watching them get bigger and bigger just as long as there not immobile/blob. I like cute fat girls so as long as there fat than it's all good, but I do like the slow and natural progression of weight gain instead of just magically big and fat in just like one day or one hour, but hey if the end result is still them being cute and fat and not too big for my tastes than i'll still be happy.
When it's slow and natural it's just more sexy and attractive to watch it's a thrill to watch them slowly gain and get bigger and then you start to wonder or imagine how big they can get or be. You can appreciate there much bigger figure all that much more as well so I tend to lean towards that option a little more. You can still appreciate them and there size if they just get big magically like that I certainly have, but it's not quite the same as slow and gradually.
I guess it also sort of depends on how the artist does everything as well.
For me, i would say a combination of both are the best at points: where the weight gain from magic/science is slow, but can speed up if a catalyst is used.
An example of of slow magic would be a Gluttony curse, where the weight gain from food is heightened to an extent (Something Chuubi uses from what I've noticed, I'm fine with being proven wrong) or on the science side a form of metabolism shutdown and appetizer enhancer.
As for option 3, if it's just 1 picture, then it's fine, but if it's a comic then you want to take it at a combination of slow and fast, but ultimately depends on what pages it happens.
i always tough about this, like a lot of WG "manga" just have the character magically or suddenly getting fat, if that's what you want to see why not just get any fat/bbw content, clearly if someone is browsing for "Weight Gain", i means they should in theory be interested in the hole process of gaining. I'm not defining these things, just saying my opinion
Agh you loveable squishy bastard! I just love the aspect of it all. But if I must choose I will choose slightly quicker over slower. But it all comes down to the "show" so to speak. Like how the person explores the growth whether it comes down to jiggling their new curves, maybe being worried about getting too big or just over joyed at being/getting bigger.