I was using lightroom 3 and 3.6 for a long time and decided to plunge in for lightroom 6 and wow i should have stayed with 3.6. ONe major complaint i have is in the development options they took out fill light and brightness options. while i have been using the new lightroom 6 i have constantly been searching for these options on multiple occassions because they are the settings that i need. And no highlights and shadows options are not good replacements for fill light. another is the way it blur and buffers the picture while editing the photo. In 3.6 it didn't have to buffer the image and and was able to show you the changes you make on the fly, so you could actually see what you where doing and the small changes you where making. Though i would have to say that i am impressed with the better load times of the catalog and when you load a photo singly.
I think a lot of people consider the new controls to be vastly superior to the old ones. Superior in the sense that you can produce higher quality images with the new tools, and in some cases vastly superior images. That certainly was my experience. It does take some learning, but I suggest you stay with it (and even search the Internet for discussion about the new tools, which collectively have the name "Process Version 2012").
Please don't think that the new Highlights is a replacement for fill light. That was never the intent. It's an entirely new set of tools and must be used differently.
Nevertheless, if you really want to use the old tools, go to the Develop Module and select Settings->Process->2010
If the develop module seems slow, try turning off the GPU Acceleration: Preferences->Performance tab, then uncheck the box