First you have to understand that I work a full time job besides making garments and costumes. That means that my sewing time ends up really being on the weekends when I am not doing something that didn't get done during the week. Plus it actually takes changing my train of thought from highly analytical to highly creative, and that is one reason why I am crazy.
When I wake up at 2 AM (which happens far too often) thinking of a dress I will sketch the dress and go back to bed. When I wake up the next morning I have to decipher that sketch. Sometimes it looks like a drunken frat boy trying to draw a stick figure. Then I have to start deciding things about the dress regarding the fabric. Sometimes the picture I have in my head is so specific that I have to search for the fabric. Once the fabric is found the I have to start the drafting process. See most people will draft the pattern while searching for fabric, but some fabric requires special accommodations and until I know what I am using I can't effectively select fabric.
See at this moment I am working on a custom gown. I found the fabric, but I have to custom dye that fabric, and that is a really specific process. Then I have to cut to work with the pattern I have drafted. This will be a long process because when I make a custom gown it is a labor of love. I am so meticulous in every little detail because I want that dress to be perfect. Now if I am making a dress from a pattern or a group of patterns then I can usually make that in a day or less because all of the hard part is dealt with already. Drafting that pattern is the LONGEST part, because I am actually not really good at that. I can do it, but not super great.
As far as alterations? I can do those as fast as any other seamstress except for the fact that I am not working at it full time. I watched the most awesome video of a dress that was in Dior show in spring of 2015. 200 HOURS!!! for one single solitary dress. That is 25-8 hour days! That means 5 regular work weeks for a single dress. This is why a custom gown costs so much more than off the rack. One of my favorite lines from Oh Brother Where Art Thou is "we ain't one at a timin' it here... we're mass communicating"
Well when it comes to a dress we are one at a timin' it! That takes longer. (LOVE THIS MOVIE)