You noticed “slowness” when using RawTherapee, but is this an issue in general on your machine and also seen when using other, graphical programs that demand a lot from it? Did you notice this after using a newer RT version? Those are some things to consider.
I’m mentioning this because a lot depends on the machine you have, more precisely: If components work well together and are balanced (and at times set up for specific tasks).
I’m not saying that RawTherapee can’t be improved, it certainly can (I want opencl, and I want it now! ), but troubleshooting performance issues isn’t all that easy to do and is often a combination of many factors.
Oh, the specs I linked to are out-of-date in one category: I have 32Gb RAM now instead of 16Gb.