Yes, sliders are really a pain. However from my experience it's much more convenient to move sliders with keyboard arrows (hover a mouse over slider and use the arrows). it still sucks, but sucks less :)
I can't speak too much to the question of speed, except to say that my nice laptop from 2012 is sluggish with darktable. My decent desktop from 2017 is fairly fluid. Neither has a graphics card, but a decent graphics card with OpenCL support makes things much more fluid. Waiting a minute for a simple operation is not something I've experienced, and suggests, as they say, that something, somewhere, is wrong.
As another commenter writes, the problem is that darktable must do a crazy lot of math to display the image -- every step of the image editing process. There is a good bit of caching along the way, but at some point, the math must be done.
Ah. wow. At the time I had a mouse it didn't work that way. Nowadays I use trackpoint only and even have no physical mouse at all. But that's good to know there's a progress. Thx.