Hi, i have a macbook pro mid 2012 originally shipped with Mavericks, upgraded to Yosemite and now i have done a full clean install of El Capitan.
I use the mac for mainly running music applications like Ableton live 9 suite, Sony sound forge and Traktor Pro, the last app in the list its recommended to not be installed until Native Instruments releases an update patch for El Capitan.
MacBook Pro (13-inch, Mid 2012)
2.5 GHz Intel Core i5
8 GB 1600 MHz DDR3 from Crucial
Intel HD Graphics 4000 1536 MB
250 GB SSD from Crucial
Trim enabled using Terminal.
7 Seconds start up under Yosemite but double the time with El Capitan or even more. I had not done the SSD upgrade while i was using Mavericks so can not speak for that matter.
I use CTRL+ALT+CMD+Eject to shut down the mac normally and it used to be a breeze.
Shut Down time is almost like windows 10, the screen desktop background stays on for a couple of seconds and after that the wheel spins a couple of times before screen goes black and you can still see the front led light is on for a bit longer.
Over all system is much faster and cooler than both previous versions of the OS in my experience and i do find changing the screen resolution form Default to the next one down the list makes the system and internet browsing much faster, pages load much smoother too but again is just what i am experiencing, some people say that the higher resolution the better.
I also hav Mac Mini running Yosemite and comparing both El Capitan feels much more responsive.
I do not think you would be able to speed up the boot time unless Apple decides to release system update some time soon in the future to address the third party SSD boot time as i am not aware if mac users with Apple SSD drives are experiencing the same.