Changing to GPU rather than CPU for compositing provides a noticeable speedup, with no downsides (that I can see). TBH, I don’t even know when it happened (currently using 4.2).
It may mean nothing on simple cases, but I have a scene with over 160 compositing operations, and on CPU I can sit and watch as it iterates through them. Meanwhile, on GPU it takes a couple of seconds.
It’s pretty well-known that everything is slower on CPU 
It seems you name is Patrick 
… and you are living under a 
Seriously… why do you think people pay over 1000$ for a graphics card and some even couple two of them ?? And maybe 250 $ for the CPU…
Yes, the small development team at Blender is slowly converting code to both run on the GPU where possible, and making it multi threaded where possible. Having folks from companies like Nvidea now helping with development is helping…
@joseph and @Okidoki , yes I’m very aware that GPU is faster. I meant when did GPU operations in the compositor become a thing?
@Matakani Thank you, that makes sense.
When?
When the GPU’s got faster than CPU’s, and the dev tools (e.g. CUDA) were easily available so certain tasks/code could be switched for performance gains. There’s more to it, but that is the basics.
Every day there is a bit more…
maybe this :
Well blow me. I usually keep my eye on these changes, but that passed me by completely.
Thank you.