
  Where does majority of the energy for the magical effect of a potion
  comes from?
  
  Does it all comes from the brewer? Or does most of it comes from the ingredients?


I don't know where does the majority of the effect come from, but at least we do know that spells are usually required. I suppose that it really depends on the potion you're working on - I don't think a spell is required for all potions.


  I know JKR said that you have to use magic at some part of the process of brewing (hence no muggle potion brewers) [what about a
  squib?]


I think it is clear, then, that it simply sucks for squibs. Which makes sense. This may contribute to explain why is Filch so inefficient identifying potions.


  If brewer provides all of the energy then why aren't NEWT students magically exhausted after every potion class?


I don't think they explicitly mention whether students were tired or not after their potion classes. They probably were, but that would most likely be because of, uh, studying hard? XD


  Hermione successfully brewed batch of Polyjuice Potion (a very potent NEWT level potion) in first semester of her second year, and
  while brewing it was very time-consuming and required a lot of
  concentration she never showed any signs of exhaustion as far as I
  remember.


Brewing that potion takes several weeks/months/whatever. She had plenty of time to rest. I think that this proves that the longer a potion takes to be made, the least exhausting it is, because most of the time it consumes is, well, waiting rather than casting spells on it too frequently. For this kind of potions, the highest requirement would probably be patience and intelligence - Hermione should do fine.

How are potions made?
I was reading this excellent site http://www.goodui.org/ when one of the points made me think 


  Repeating your call to action is a strategy that is more applicable to
  longer pages, or repeating across numerous pages. Surely you don't
  want to have your offer displayed 10 times all on the same screen and
  frustrate people. However, long pages are becoming the norm and the
  idea of squeezing everything "above the fold" is fading. It doesn't
  hurt to have one soft actionable item at the top, and another
  prominent one at the bottom. When people reach the bottom, they pause
  and think what to do next - a potential solid place to make an offer
  or close a deal.
  
  


While I think this is a good solution in which you have a reasonably sized page where you can reach the top or bottom in three-four scrolls, what if you have a really long page. Do we really know if users will scroll all the way to the bottom and then make a decision ? In that case wont it be better to have a floating call to action button which allows them to do an action at any point of time ? 

Please do note I am not suggesting this button should hide any text or  content but it would float at the side where it is visible but not in the way.

Please note I did see these questions and go through the answers in detail

Fixed/floating button on mobile app

Should I add a floating, fixed feedback button on the pages of my website?

However they don't apply to my question specifically.
