True that gems often do more than I need, but should I?
1) Write my own authentication code
2) Write my own ical feed code (just added this in about 2 hours using ri_cal, which has way more features than I need)
3) Write my own code to inline css in email
4) Write my own Facebook API library
5) Write my own admin interface
6) Write my own SCSS and HAML processors
7) Write my own file attachment and processing code
8) Write my own background processing library
9) Write my own CSS/JS compression library
You get the point.
I use gems that save me tons of time. If not for gems, I would not be using Rails.
That's only 9 libraries, and that shouldn't be enough to cause your load times to reach 20-30 seconds.
Also, since you asked :-) I would question #5. Auto admin libraries (and this applies to multiple languages and frameworks) are generally heavy, too general and have code hidden away in the library. In contrast, banging out crud views and controllers in a locked-down namespace is easy, only relies on same dependencies as the rest of the app, can be customized for the specific business needs and all the code is in one obvious place.
It would take me many days to create and test an admin with multiple objects, even a simple one, and then I have to maintain it. I install rails_admin and I've got what I need. This is for internal testing and maintenance. I don't care if it's heavy.
I suppose your logic makes sense if you're gainfully employed, but I bang this stuff out for myself and clients. A 90% solution with one line of code sure beats weeks of work.
You get the point.
I use gems that save me tons of time. If not for gems, I would not be using Rails.